[Suport][cmake] Change LLVM_INTEGRATED_CRT_ALLOC to a path instead of a boolean

[aganea]

Summary

Fix issue, as pointed out by Russell: https://reviews.llvm.org/D71786#2270702

[russell.gallop]

One small comment. Apart from that, LGTM.

Thanks

llvm/CMakeLists.txt

516

I think it might make more sense to put all of the new LLVM_INTEGRATED_CRT_ALLOC code further up now, such as here, so it is not under "Define options to control the inclusion..." section which is (almost entirely) boolean options to enable build targets.

llvm/CMakeLists.txt

if (MSVC_IDE OR XCODE)
   set(LIT_ARGS_DEFAULT "${LIT_ARGS_DEFAULT} --no-progress-bar")
 endif()
 set(LLVM_LIT_ARGS "${LIT_ARGS_DEFAULT}" CACHE STRING "Default options for lit")
 
 # On Win32 hosts, provide an option to specify the path to the GnuWin32 tools.
 if( WIN32 AND NOT CYGWIN )
   set(LLVM_LIT_TOOLS_DIR "" CACHE PATH "Path to GnuWin32 tools")
 endif()
 
 # Define options to control the inclusion and default build behavior for
 # components which may not strictly be necessary (tools, examples, and tests).
 #
 # This is primarily to support building smaller or faster project files.
 option(LLVM_INCLUDE_TOOLS "Generate build targets for the LLVM tools." ON)
 option(LLVM_BUILD_TOOLS
   "Build the LLVM tools. If OFF, just generate build targets." ON)
 
set(LLVM_INSTALL_OCAMLDOC_HTML_DIR "share/doc/llvm/ocaml-html"
     CACHE STRING "OCamldoc-generated HTML documentation install directory")
 
 option (LLVM_BUILD_EXTERNAL_COMPILER_RT
   "Build compiler-rt as an external project." OFF)
 
 option (LLVM_VERSION_PRINTER_SHOW_HOST_TARGET_INFO
   "Show target and host info when tools are invoked with --version." ON)
 
-option(LLVM_INTEGRATED_CRT_ALLOC "Replace the Windows CRT allocator with any of {rpmalloc|mimalloc|snmalloc}. Only works with /MT enabled." OFF)
+set(LLVM_INTEGRATED_CRT_ALLOC "" CACHE PATH "Replace the Windows CRT allocator with any of {rpmalloc|mimalloc|snmalloc}. Only works with /MT enabled.")
 if(LLVM_INTEGRATED_CRT_ALLOC)
   if(NOT WIN32)
     message(FATAL_ERROR "LLVM_INTEGRATED_CRT_ALLOC is only supported on Windows.")
   endif()
   if(LLVM_USE_SANITIZER)
     message(FATAL_ERROR "LLVM_INTEGRATED_CRT_ALLOC cannot be used along with LLVM_USE_SANITIZER!")
   endif()
   if(CMAKE_BUILD_TYPE AND uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")