[CMake] Improve GetHostTriple for Windows/ARM configurations

[mstorsjo]

Summary

In MSVC mode, CMake provides CMAKE_C_COMPILER_ARCHITECTURE_ID,
which should be accurate for what the compiler produces. Keep
the old CMAKE_C_SIZEOF_VOID_P based fallback in case this
doesn't provide matches.

In GCC/MinGW mode, CMake doesn't provide that. As we're trying to
guess the default host architecture, use CMAKE_HOST_SYSTEM_PROCESSOR
in combination with the existing CMAKE_C_SIZEOF_VOID_P check to
estimate whether it's x86/x86_64/arm/aarch64.

I made a MR to upstream CMake to make it provide
CMAKE_<LANG>_COMPILER_ARCHITECTURE_ID for GCC and non-MSVC Clang
configurations too, in
https://gitlab.kitware.com/cmake/cmake/-/merge_requests/6432,
but I wouldn't want to start relying on that right away.

[phosek]

LGTM

llvm/cmake/modules/GetHostTriple.cmake

 # Returns the host triple.
 # Invokes config.guess
 
 function( get_host_triple var )
   if( MSVC )
-    if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
+    if( CMAKE_C_COMPILER_ARCHITECTURE_ID MATCHES "ARM64.*" )
+      set( value "aarch64-pc-windows-msvc" )
+    elseif( CMAKE_C_COMPILER_ARCHITECTURE_ID MATCHES "ARM.*" )
+      set( value "armv7-pc-windows-msvc" )
+    elseif( CMAKE_C_COMPILER_ARCHITECTURE_ID STREQUAL "x64" )
+      set( value "x86_64-pc-windows-msvc" )
+    elseif( CMAKE_C_COMPILER_ARCHITECTURE_ID STREQUAL "X86" )
+      set( value "i686-pc-windows-msvc" )
+    elseif( CMAKE_SIZEOF_VOID_P EQUAL 8 )
       set( value "x86_64-pc-windows-msvc" )
     else()
       set( value "i686-pc-windows-msvc" )
     endif()
   elseif( MINGW AND NOT MSYS )
+    # CMake doesn't provide COMPILER_ARCHITECTURE_ID for Clang/GCC,
+    # but it does for MSVC.
+    if( CMAKE_HOST_SYSTEM_PROCESSOR MATCHES "ARM.*" )
+      if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
+        set( value "aarch64-w64-windows-gnu" )
+      else()
+        set( value "armv7-pc-windows-gnu" )
+      endif()
+    else()
       if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
         set( value "x86_64-w64-windows-gnu" )
       else()
         set( value "i686-pc-windows-gnu" )
       endif()
+    endif()
   elseif( CMAKE_SYSTEM_NAME MATCHES "OS390" )
     set( value "s390x-ibm-zos" )
   elseif( CMAKE_SYSTEM_NAME STREQUAL AIX )
     # We defer to dynamic detection of the host AIX version.
     if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
       set( value "powerpc64-ibm-aix" )
     else()
       set( value "powerpc-ibm-aix" )