[cmake] Centralize LLVM_ENABLE_WARNINGS option

Configure default value of `LLVM_ENABLE_WARNINGS` in `HandleLLVMOptions.cmake`.

`LLVM_ENABLE_WARNINGS` is documented as ON by default, but `HandleLLVMOptions` assumes the default has been set somewhere else. If it has not been explicitly set, then `HandleLLVMOptions` implicitly uses OFF as a default.

This removes the various `option()` declarations in favor of a single declaration in `HandleLLVMOptions`. This will prevent the unwanted use of `-w` that is mentioned in a couple of the comments.

Reviewed By: DavidTruby, #libunwind, JDevlieghere, compnerd

Differential Revision: https://reviews.llvm.org/D87243

CMakeLists.txt

@@ -399,7 +399,6 @@ set(LLVM_TARGETS_TO_BUILD
 list(REMOVE_DUPLICATES LLVM_TARGETS_TO_BUILD)
 
 option(LLVM_ENABLE_PIC "Build Position-Independent Code" ON)
-option(LLVM_ENABLE_WARNINGS "Enable compiler warnings." ON)
 option(LLVM_ENABLE_MODULES "Compile with C++ modules enabled." OFF)
 if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
   option(LLVM_ENABLE_MODULE_DEBUGGING "Compile with -gmodules." ON)

cmake/modules/HandleLLVMOptions.cmake

@@ -409,6 +409,8 @@ elseif(MINGW) # FIXME: Also cygwin?
   endif()
 endif()
 
+option(LLVM_ENABLE_WARNINGS "Enable compiler warnings." ON)
+
 if( MSVC )
   include(ChooseMSVCCRT)
 

cmake/modules/LLVMConfig.cmake.in

@@ -36,6 +36,8 @@ set(TARGET_TRIPLE "@TARGET_TRIPLE@")
 
 set(LLVM_ABI_BREAKING_CHECKS @LLVM_ABI_BREAKING_CHECKS@)
 
+set(LLVM_ENABLE_WARNINGS @LLVM_ENABLE_WARNINGS@)
+
 set(LLVM_ENABLE_EXPENSIVE_CHECKS @LLVM_ENABLE_EXPENSIVE_CHECKS@)
 
 set(LLVM_ENABLE_ASSERTIONS @LLVM_ENABLE_ASSERTIONS@)

runtimes/CMakeLists.txt

@@ -105,10 +105,6 @@ if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
   # Avoid checking whether the compiler is working.
   set(LLVM_COMPILER_CHECKED ON)
 
-  # Enable warnings, otherwise -w gets added to the cflags by HandleLLVMOptions
-  # resulting in unjustified successes by check_cxx_compiler_flag.
-  set(LLVM_ENABLE_WARNINGS ON)
-
   # Handle common options used by all runtimes.
   include(AddLLVM)
   include(HandleLLVMOptions)