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

[Kajetan Puchalski via llvm-commits]

================
@@ -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")
----------------
mrkajetanp wrote:

The oldest clang I have on hand is 14 and it does support the flag already, not sure about the earlier ones.

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