[CMake] Support using precompiled headers with ccache in flang (#136856)

In order for precompiled headers to work with ccache, a specific flag
needs to be passed to the compiler and ccache's sloppiness configuration
option needs to be set appropriately.

Due to issues with configuring CMake on certain Windows platforms, set
the required ccache option only on non-Windows systems for the time
being.

-----

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>

flang/CMakeLists.txt

diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index 76eb132..ed02fad 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -452,6 +452,10 @@
    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} -fno-semantic-interposition")
   endif()
 
+  # GCC requires this flag in order for precompiled headers to work with ccache
+  if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpch-preprocess")
+  endif()
 endif()
 
 # Clang on Darwin enables non-POSIX extensions by default, which allows the
@@ -462,6 +466,11 @@
   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" AND NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Xclang -fno-pch-timestamp")
+endif()
+
 list(REMOVE_DUPLICATES CMAKE_CXX_FLAGS)
 
 # Determine HOST_LINK_VERSION on Darwin.

llvm/CMakeLists.txt

diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 693cb08..d47ac3c 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -273,7 +273,7 @@
   if(CCACHE_PROGRAM)
     set(LLVM_CCACHE_MAXSIZE "" CACHE STRING "Size of ccache")
     set(LLVM_CCACHE_DIR "" CACHE STRING "Directory to keep ccached data")
-    set(LLVM_CCACHE_PARAMS "CCACHE_CPP2=yes CCACHE_HASHDIR=yes"
+    set(LLVM_CCACHE_PARAMS "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros"
         CACHE STRING "Parameters to pass through to ccache")
 
     if(NOT CMAKE_SYSTEM_NAME MATCHES "Windows")
@@ -287,7 +287,7 @@
       set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
     else()
       if(LLVM_CCACHE_MAXSIZE OR LLVM_CCACHE_DIR OR
-         NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes")
+         NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros")
         message(FATAL_ERROR "Ccache configuration through CMake is not supported on Windows. Please use environment variables.")
       endif()
       # RULE_LAUNCH_COMPILE should work with Ninja but currently has issues