[flang] [llvm] [CMake] Fix using precompiled headers with ccache (PR #131397)

[Michał Górny via llvm-commits]

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@@ -446,6 +446,11 @@ if (APPLE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
 endif()
 
+# Clang requires this flag in order for precompiled headers to work with ccache.
+if (CMAKE_CXX_COMPILER_ID MATCHES Clang)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -fno-pch-timestamp")
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mgorny wrote:

Sure — but the point is, I wouldn't even have realized that I need to set something special. Not a big deal for me, just pointing out that PCHs aren't that common and people don't have awareness that clang requires special flags for, well, reproducible builds.

https://github.com/llvm/llvm-project/pull/131397