[llvm] [CMake] Make precompiled headers opt-in for ccache builds (PR #141927)

[Aiden Grossman via llvm-commits]

================
@@ -289,6 +289,12 @@ if(LLVM_CCACHE_BUILD)
     set(LLVM_CCACHE_PARAMS "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros"
         CACHE STRING "Parameters to pass through to ccache")
 
+    # Using precompiled headers with ccache builds can result in intermittent CI failures
+    set(LLVM_CCACHE_PCH OFF CACHE BOOL "Use precompiled headers in ccache builds")
----------------
boomanaiden154 wrote:

New flags should be documented in `llvm/docs/CMake.rst`.

https://github.com/llvm/llvm-project/pull/141927