Add IntelliSense mode and support for emscripten / web assembly

Environment

• OS and version: macOS 12.5
• VS Code: 1.70.2
• C/C++ extension: 1.11.5
• GDB / LLDB version: N/A

Bug Summary and Steps to Reproduce

IntelliSense does not work for Emscripten. C++ Tool will query the default compiler target, but would not recognize it:

Querying compiler's default target using command line: "/Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/em++" -dumpmachine
Compiler returned default target value: wasm32-unknown-emscripten
Unhandled default compiler target value detected: wasm32-unknown-emscripten

And then it will go on and query the headers with the wrong target specified (in this case, ARM):

Compiler query command line: /Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/em++ -std=gnu++17 --target=aarch64-arm-none-eabi -Wp,-v -fno-blocks -E -dM -x c++ /dev/null

This would cause Emscripten's clang to use the wrong relative sysroot directory for header search, thus breaking IntelliSense.

tommyli@HOST ~ % em++ -std=gnu++17 --target=aarch64-arm-none-eabi -Wp,-v -fno-blocks -E -dM -x c++ /dev/null
clang -cc1 version 16.0.0 based upon LLVM 16.0.0git default target x86_64-apple-darwin21.6.0
ignoring nonexistent directory "/Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/usr/local/include"
ignoring nonexistent directory "/Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/usr/include"
#include "..." search starts here:
#include <...> search starts here:
 /Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/include/SDL
 /Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/include/compat
 /Users/tommyli/Documents/emscripten-core/emsdk/upstream/lib/clang/16.0.0/include
End of search list.

Dropping --target would make cm++ finding the sysroot headers correctly:

tommyli@HOST ~ % em++ -std=gnu++17 -Wp,-v -fno-blocks -E -dM -x c++ /dev/null                               
clang -cc1 version 16.0.0 based upon LLVM 16.0.0git default target x86_64-apple-darwin21.6.0
ignoring nonexistent directory "/Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
 /Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/include/SDL
 /Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/include/compat
 /Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
 /Users/tommyli/Documents/emscripten-core/emsdk/upstream/lib/clang/16.0.0/include
 /Users/tommyli/Documents/emscripten-core/emsdk/upstream/emscripten/cache/sysroot/include

Debugger Configurations

N/A

Debugger Logs

N/A

Other Extensions

No response

Additional Information

No response

Start by reproducing the compiler-target query with em++ and compare the header search results with and without --target, as shown in the issue. Trace how the reported wasm32-unknown-emscripten target is handled and verify that IntelliSense recognizes it and queries the correct Emscripten sysroot headers.