Ship the Windows toolchain with LLVM built as a DLL

[Steelskin]

Description

This task tracks the work needed to ship the Windows toolchain with LLVM built as a DLL. This can unlock some improvements:

• Build the toolchain faster since we will only be building one huge binary target (LLVM.dll), the others will be much smaller and depend on LLVM.dll
• Ease support for thin LTO/PGO. We can focus on performance improvements in LLVM.dll rather than having to do the work for every binary.
• Reduce the toolchain installation build size. Currently, almost every executable binary that we ship packages a full copy of LLVM. Splitting out LLVM.dll could save around 500 MiB of installed size.

Additional information

The following build flags need to be added to the compiler build in build.ps1:

    LLVM_BUILD_LLVM_DYLIB = "YES";
    LLVM_DYLIB_EXPORT_INLINES = "YES";
    LLVM_BUILD_LLVM_DYLIB_VIS = "YES";
    LLVM_LINK_LLVM_DYLIB = "YES";
    CLANG_LINK_CLANG_DYLIB = "NO";