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

[Kajetan Puchalski 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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mrkajetanp wrote:

My understanding is that clang will look at the file size and the timestamp in order to work out whether it needs to be recompiled or not. Without the timestamp, it's just left with having to rely on whether the size has changed or not. Looking at [the original commit message](https://github.com/llvm/llvm-project/commit/533a893fa126d8a9662d836ad28f51afa1d2e74a) it seems as if the flag was added after people realised the pch timestamps were breaking distributed build systems. So maybe it's not the default because it was done after the fact. Though I don't have any special insight there outside of just inferring things.

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