[clang] Assertion `!IsDependent && "explicit instantiation of dependent name not yet handled"' failed.

Reproducer:
https://godbolt.org/z/TKnd4afqG
```cpp
namespace foo {
template struct S {};
} // namespace foo

template struct foo::S::bar;
```

Backtrace:
```console
clang++: /root/llvm-project/llvm/tools/clang/lib/Sema/SemaTemplate.cpp:10602: clang::DeclResult clang::Sema::ActOnExplicitInstantiation(clang::Scope*, clang::SourceLocation, clang::SourceLocation, unsigned int, clang::SourceLocation, clang::CXXScopeSpec&, clang::IdentifierInfo*, clang::SourceLocation, const clang::ParsedAttributesView&): Assertion `!IsDependent && "explicit instantiation of dependent name not yet handled"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and dumped files.
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
1. :5:33: current parser token ';'
#0 0x0000000004413f18 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4413f18)
#1 0x0000000004410dc4 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4410dc4)
#2 0x00000000044113fc llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44113fc)
#3 0x000000000434eec8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x000076ee08645330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x000076ee0869ec0c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9ec0c)
#6 0x000076ee0864527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x000076ee086288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x000076ee0862881b (/lib/x86_64-linux-gnu/libc.so.6+0x2881b)
#9 0x000076ee0863b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#10 0x00000000077e6049 clang::Sema::ActOnExplicitInstantiation(clang::Scope*, clang::SourceLocation, clang::SourceLocation, unsigned int, clang::SourceLocation, clang::CXXScopeSpec&, clang::IdentifierInfo*, clang::SourceLocation, clang::ParsedAttributesView const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x77e6049)
#11 0x0000000006ddc60f clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ddc60f)
#12 0x0000000006daadcc clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6daadcc)
#13 0x0000000006e83909 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e83909)
#14 0x0000000006e8449c clang::Parser::ParseExplicitInstantiation(clang::DeclaratorContext, clang::SourceLocation, clang::SourceLocation, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e8449c)
#15 0x0000000006e916e8 clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e916e8)
#16 0x0000000006dbeb50 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dbeb50)
#17 0x0000000006d75530 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d75530)
#18 0x0000000006d76445 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d76445)
#19 0x0000000006d5281a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d5281a)
#20 0x0000000004f213c8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f213c8)
#21 0x0000000005251378 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5251378)
#22 0x00000000051dec0d clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x51dec0d)
#23 0x000000000532566d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x532566d)
#24 0x0000000000d8fbcc cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd8fbcc)
#25 0x0000000000d868ca ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#26 0x0000000000d86a4d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#27 0x0000000004f99ee9 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#28 0x000000000434f2c4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x434f2c4)
#29 0x0000000004f9aa55 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#30 0x0000000004f525c2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f525c2)
#31 0x0000000004f56f27 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f56f27)
#32 0x0000000004f61675 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f61675)
#33 0x0000000000d8c0ba clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd8c0ba)
#34 0x0000000000cc9fea main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcc9fea)
#35 0x000076ee0862a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#36 0x000076ee0862a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#37 0x0000000000d86275 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd86275)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
Compiler returned: 134
```

Reproduce the crash with the linked Compiler Explorer example, then inspect llvm/tools/clang/lib/Sema/SemaTemplate.cpp around ActOnExplicitInstantiation and the assertion at line 10602. Determine the expected handling for this dependent explicit instantiation; done means the reproducer no longer triggers an assertion and the behavior is covered by a regression test.

Written by the indexing model from the issue text.