[ccache] Add Windows support

[NathanielMcVicar]

Summary

Windows builds can't receive environment variables on the command line or make use of RULE_LAUNCH_COMPILE with ccache.

[Ericson2314]

I know nothing about ccache on Windows, but if it didn't work before, I think it is pretty clear that this at least won't cause any sort of regression.

[Ericson2314]

So ideally someone else reviews this, but if not, +1 from me as at least no worse than before.

[stella.stamenova]

LGTM

llvm/CMakeLists.txt

195

Can you add a comment that we can't use RULE_LAUNCH_COMPILE on Windows even though in theory it should be able to work?

[NathanielMcVicar]

Add comment for RULE_LAUNCH_COMPILE on Windows

[NathanielMcVicar]

Fix merge base

llvm/CMakeLists.txt

if(NOT CMAKE_SYSTEM_NAME MATCHES "Windows")
         set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")
       endif()
       set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
     else()
       if(LLVM_CCACHE_MAXSIZE OR LLVM_CCACHE_DIR OR
          NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes")
         message(FATAL_ERROR "Ccache configuration through CMake is not supported on Windows. Please use environment variables.")
       endif()
+      # RULE_LAUNCH_COMPILE should work with Ninja but currently has issues
+      # with cmd.exe and some MSVC tools other than cl.exe
       set(CMAKE_C_COMPILER_LAUNCHER ${CCACHE_PROGRAM})
       set(CMAKE_CXX_COMPILER_LAUNCHER ${CCACHE_PROGRAM})     
     endif()
   else()
     message(FATAL_ERROR "Unable to find the program ccache. Set LLVM_CCACHE_BUILD to OFF")
   endif()
 endif()