[clang] Assertion `From->getType()->isIntegerType()' failed.

Reproducer:
https://godbolt.org/z/WTbnq6jdr
```cpp
#define CONV(TYPE, NAME) \
_Complex int NAME##_to_CI(TYPE a) { return a; }

CONV(short _Fract, sf);
```

Backtrace:
```console
clang++: /root/llvm-project/llvm/tools/clang/lib/Sema/SemaExprCXX.cpp:5111: clang::ExprResult clang::Sema::PerformImplicitConversion(clang::Expr*, clang::QualType, const clang::StandardConversionSequence&, clang::AssignmentAction, clang::CheckedConversionKind): Assertion `From->getType()->isIntegerType()' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -ffixed-point
1. :4:1 :2:47>: current parser token ';'
2. :4:1 :2:37>: parsing function body 'sf_to_CI'
3. :4:1 :2:37>: in compound statement ('{}')
#0 0x0000000004214d18 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4214d18)
#1 0x0000000004212144 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4212144)
#2 0x0000000004156348 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x000078bc42e42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x000078bc42e969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#5 0x000078bc42e42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#6 0x000078bc42e287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#7 0x000078bc42e2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#8 0x000078bc42e39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#9 0x0000000006f42367 (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f42367)
#10 0x0000000006f4279a clang::Sema::PerformImplicitConversion(clang::Expr*, clang::QualType, clang::ImplicitConversionSequence const&, clang::AssignmentAction, clang::CheckedConversionKind) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f4279a)
#11 0x0000000006fcf6f6 clang::InitializationSequence::Perform(clang::Sema&, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef, clang::QualType*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fcf6f6)
#12 0x0000000006fd2342 clang::Sema::PerformCopyInitialization(clang::InitializedEntity const&, clang::SourceLocation, clang::ActionResult, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fd2342)
#13 0x00000000072506b4 clang::Sema::PerformMoveOrCopyInitialization(clang::InitializedEntity const&, clang::Sema::NamedReturnInfo const&, clang::Expr*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x72506b4)
#14 0x0000000007268354 clang::Sema::BuildReturnStmt(clang::SourceLocation, clang::Expr*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7268354)
#15 0x0000000007268f28 clang::Sema::ActOnReturnStmt(clang::SourceLocation, clang::Expr*, clang::Scope*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7268f28)
#16 0x0000000006a0e66a clang::Parser::ParseReturnStatement() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a0e66a)
#17 0x0000000006a1299a clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a1299a)
#18 0x0000000006a139c9 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a139c9)
#19 0x0000000006a1c156 clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a1c156)
#20 0x0000000006a1c96a clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a1c96a)
#21 0x000000000692343b clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x692343b)
#22 0x00000000069598ad clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69598ad)
#23 0x0000000006916bee clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6916bee)
#24 0x000000000691738f clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x691738f)
#25 0x000000000691ec43 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x691ec43)
#26 0x000000000691fb55 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x691fb55)
#27 0x0000000006920000 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6920000)
#28 0x00000000069011a3 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69011a3)
#29 0x0000000004b5d4e8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b5d4e8)
#30 0x0000000004e4a685 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e4a685)
#31 0x0000000004dcb73e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4dcb73e)
#32 0x0000000004f41c2d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f41c2d)
#33 0x0000000000dce380 cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdce380)
#34 0x0000000000dc4eea ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#35 0x0000000000dc506d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#36 0x0000000004bc6a79 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#37 0x00000000041567e4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41567e4)
#38 0x0000000004bc708f clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#39 0x0000000004b87fd2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b87fd2)
#40 0x0000000004b88f7e clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b88f7e)
#41 0x0000000004b90695 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b90695)
#42 0x0000000000dca8a9 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdca8a9)
#43 0x0000000000c79c64 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc79c64)
#44 0x000078bc42e29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#45 0x000078bc42e29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#46 0x0000000000dc4985 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdc4985)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

Reproduce the assertion with the linked Compiler Explorer example and the shown `-ffixed-point` compiler invocation. Start in `llvm/tools/clang/lib/Sema/SemaExprCXX.cpp` at `Sema::PerformImplicitConversion` around line 5111, then trace the return initialization path shown in the backtrace. Done means this input no longer aborts with the assertion.

Written by the indexing model from the issue text.