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

[Tom Eccles 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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tblah wrote:

Another thought, how recent is clang's PCH support? Is -fno-pch-timestamp supported on all versions of clang supported by LLVM?

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