[CMake] Allow overriding MSVC_DIA_SDK_DIR via CMake

This eases using it in cross-msvc setups.

Differential Revision: https://reviews.llvm.org/D69239

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375485 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/config-ix.cmake

@@ -474,7 +474,8 @@ if( MSVC )
   set(strdup "_strdup")
 
   # See if the DIA SDK is available and usable.
-  set(MSVC_DIA_SDK_DIR "$ENV{VSINSTALLDIR}DIA SDK")
+  set(MSVC_DIA_SDK_DIR "$ENV{VSINSTALLDIR}DIA SDK" CACHE PATH
+      "Path to the DIA SDK")
 
   # Due to a bug in MSVC 2013's installation software, it is possible
   # for MSVC 2013 to write the DIA SDK into the Visual Studio 2012