GH-131473: tweak PreferredToolArchitecture for bundled clang-cl

[chris-eibl]

This PR tweaks PreferredToolArchitecture so that the clang-cl bundled with VS can do PGO builds by just invoking

build.bat -p x64 --pgo "/p:PlatformToolset=ClangCL"

or

build.bat -p Win32 --pgo "/p:PlatformToolset=ClangCL"

without the need to add the appropriate PreferredToolArchitecture like

build.bat -p x64 --pgo "/p:PlatformToolset=ClangCL" "/p:PreferredToolArchitecture=x64"

or

build.bat -p Win32 --pgo "/p:PlatformToolset=ClangCL" "/p:PreferredToolArchitecture=x86"

Otherwise, the respective clang_rt.profile.lib is not found.

IMHO this is convenient for users who just want to use the bundled version of clang-cl.

Users who like to use a different version will have to use an appropriate bitness of clang-cl and point LLVMInstallDir to it.
No need for PreferredToolArchitecture in this case, anyway. Hence, the "tweaking" is only done when LLVMInstallDir is empty,
so that <VS install path>\Community\MSBuild\Microsoft\VC\v160\Microsoft.Cpp.ClangCl.Common.props:

    <_DefaultLLVMInstallDir Condition="'$(_DefaultLLVMInstallDir)' == '' AND '$(PreferredToolArchitecture)' == 'arm64'">$(VsInstallRoot)\VC\Tools\Llvm\ARM64</_DefaultLLVMInstallDir>
    <_DefaultLLVMInstallDir Condition="'$(_DefaultLLVMInstallDir)' == '' AND '$(PreferredToolArchitecture)' == 'x64'">$(VsInstallRoot)\VC\Tools\Llvm\x64</_DefaultLLVMInstallDir>
    <_DefaultLLVMInstallDir Condition="'$(_DefaultLLVMInstallDir)' == '' AND '$(PreferredToolArchitecture)' != 'x64'">$(VsInstallRoot)\VC\Tools\Llvm</_DefaultLLVMInstallDir>
    <LLVMInstallDir Condition="'$(LLVMInstallDir)' == ''">$(_DefaultLLVMInstallDir)</LLVMInstallDir>

will set the the LLVMInstallDir to the correct bundled installation folder.

For details please see the issue.

• Issue: clang-cl on Windows still needs PreferredToolArchitecture #131473