[llvm] Enable the use of SCCache for debug builds on Windows (PR #82371)

[Dave Abrahams via llvm-commits]

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@@ -10,3 +10,9 @@ endif()
 if(POLICY CMP0116)
   cmake_policy(SET CMP0116 OLD)
 endif()
+
++# Enable debug info compatible with SCCache
----------------
dabrahams wrote:

OK, following the precedent set by the other policies, I'll add CMake's own summary: [MSVC debug information format flags are selected by an abstraction](https://cmake.org/cmake/help/latest/policy/CMP0141.html).  I find it barely comprehensible, and don't find it very useful.  A more accessible explanation is that without the flag, `cl` complains of what looks like multiple simultaneous writes to the same .pdb file, so maybe I'll add that too.

https://github.com/llvm/llvm-project/pull/82371