[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:

> Only when ccache and precompiled headers are enabled, no?

I added a check to make sure that precompiled headers are not disabled.
For ccache, there are setups people will be using that use ccache without using LLVM_CCACHE_BUILD, whether it be trickery with setting PATH or overriding CMAKE_CXX_COMPILER_LAUNCHER by hand or whatnot.
On that account I think it's not feasible to reliably check whether someone actually is using ccache or not. I don't think there's any harm in leaving the flag in even when it's not strictly necessary and it'll avoid problems with these exotic setups.

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