[CMake] Support per-target compiler and linker flags

[thevinster]

Summary

CMAKE_{C/CXX}_FLAGS affects all targets in LLVM. This is
undesirable in situations where we do not want to apply
every optimization flag to every target for faster build speeds.
For example, we can selectively apply -flto=thin flags to
binaries that we care about optimizing and keep other tools
in the distribution on the non-optimized version. The pattern
follows a similar approach to how multi-distributions are created.
The example of the thinLTO above can be done by doing the
following:

set(LLVM_CUSTOM_BINARIES clang lld CACHE STRING "List of targets to apply custom flags")
set(LLVM_clang_COMPILER_FLAGS -flto=thin CACHE STRING "Custom compiler flags to clang")
set(LLVM_lld_COMPILER_FLAGS -flto=thin CACHE STRING "Custom compiler flags to lld")

There's probably other applications where this could be used
(e.g. avoid optimizing host tools for build speed improvement etc.).
I've generalized this so that users can apply their desired flags to
targets that are generated by llvm_add_library and add_llvm_executable.

[thevinster]

Would love to get some feedback whether there's interest in this or if there's a better way to solve the build speed issue.

[thevinster]

Will resubmit on Github along with more testing results.

llvm/cmake/modules/AddLLVM.cmake

if(LLVM_COMMON_DEPENDS)
     add_dependencies(${name} ${LLVM_COMMON_DEPENDS})
     # Add dependencies also to objlibs.
     # CMake issue 14747 --  add_dependencies() might be ignored to objlib's user.
     foreach(objlib ${objlibs})
       add_dependencies(${objlib} ${LLVM_COMMON_DEPENDS})
     endforeach()
   endif()
 
+  if (${name} IN_LIST LLVM_CUSTOM_BINARIES)
+    if (NOT LLVM_${name}_COMPILER_FLAGS AND NOT LLVM_${name}_SHARED_LINKER_FLAGS AND NOT LLVM_${name}_MODULE_LINKER_FLAGS)
+      message(FATAL_ERROR "At least one of compiler or linker flags must be provided when specifying LLVM_CUSTOM_BINARIES")
+    else()
+      if (LLVM_${name}_COMPILER_FLAGS)
+        message(STATUS "Applying ${LLVM_${name}_COMPILER_FLAGS} to ${name}")
+        target_compile_options(${name} PRIVATE ${LLVM_${name}_COMPILER_FLAGS})
+        # For binaries that link with static libraries, recursively apply the flag to those targets.
+        get_target_property(target_libraries ${name} LINK_LIBRARIES)
+        foreach(lib ${target_libraries})
+          if(TARGET ${lib})
+            message(STATUS "Applying ${LLVM_${name}_COMPILER_FLAGS} to ${lib}")
+            target_compile_options(${lib} PRIVATE ${LLVM_${name}_COMPILER_FLAGS})
+          else()
+            message(STATUS "Target ${lib} does not exist in the project. Skipping...")
+          endif()
+        endforeach()
+      endif()
+      if (ARG_SHARED)
+        if (LLVM_${name}_SHARED_LINKER_FLAGS)
+          message(STATUS "Applying ${LLVM_${name}_SHARED_LINKER_FLAGS} to ${name}")
+          target_link_options(${name} PRIVATE ${LLVM_${name}_SHARED_LINKER_FLAGS})
+        endif()
+      endif()
+      if (ARG_MODULE)
+        if (LLVM_${name}_MODULE_LINKER_FLAGS)
+          message(STATUS "Applying ${LLVM_${name}_MODULE_LINKER_FLAGS} to ${name}")
+          target_link_options(${name} PRIVATE ${LLVM_${name}_MODULE_LINKER_FLAGS})
+        endif()
+      endif()
+    endif()
+  endif()
+
   if(ARG_SHARED OR ARG_MODULE)
     llvm_externalize_debuginfo(${name})
     llvm_codesign(${name} ENTITLEMENTS ${ARG_ENTITLEMENTS} BUNDLE_PATH ${ARG_BUNDLE_PATH})
   endif()
   # clang and newer versions of ninja use high-resolutions timestamps,
   # but older versions of libtool on Darwin don't, so the archive will
   # often get an older timestamp than the last object that was added
   # or updated.  To fix this, we add a custom command to touch archive
macro(add_llvm_executable name)
   llvm_config( ${name} ${USE_SHARED} ${LLVM_LINK_COMPONENTS} )
   if( LLVM_COMMON_DEPENDS )
     add_dependencies( ${name} ${LLVM_COMMON_DEPENDS} )
     foreach(objlib ${obj_name})
       add_dependencies(${objlib} ${LLVM_COMMON_DEPENDS})
     endforeach()
   endif( LLVM_COMMON_DEPENDS )
 
+  if (${name} IN_LIST LLVM_CUSTOM_BINARIES)
+    if (NOT LLVM_${name}_COMPILER_FLAGS AND NOT LLVM_${name}_EXE_LINKER_FLAGS)
+      message(FATAL_ERROR "At least one of compiler or linker flags must be provided when specifying LLVM_CUSTOM_BINARIES")
+    else()
+      if (LLVM_${name}_COMPILER_FLAGS)
+        message(STATUS "Applying ${LLVM_${name}_COMPILER_FLAGS} to ${name}")
+        target_compile_options(${name} PRIVATE ${LLVM_${name}_COMPILER_FLAGS})
+        # For binaries that link with static libraries, recursively apply the flag to those targets.
+        get_target_property(target_libraries ${name} LINK_LIBRARIES)
+        foreach(lib ${target_libraries})
+          if(TARGET ${lib})
+            message(STATUS "Applying ${LLVM_${name}_COMPILER_FLAGS} to ${lib}")
+            target_compile_options(${lib} PRIVATE ${LLVM_${name}_COMPILER_FLAGS})
+          else()
+            message(STATUS "Target ${lib} does not exist in the project. Skipping...")
+          endif()
+        endforeach()
+      endif()
+      if (LLVM_${name}_EXE_LINKER_FLAGS)
+        message(STATUS "Applying ${LLVM_${name}_EXE_LINKER_FLAGS} to ${name}")
+        target_link_options(${name} PRIVATE ${LLVM_${name}_EXE_LINKER_FLAGS})
+      endif()
+    endif()
+  endif()
+
   if(NOT ARG_IGNORE_EXTERNALIZE_DEBUGINFO)
     llvm_externalize_debuginfo(${name})
   endif()
   if (LLVM_PTHREAD_LIB)
     # libpthreads overrides some standard library symbols, so main
     # executable must be linked with it in order to provide consistent
     # API for all shared libaries loaded by this executable.
     target_link_libraries(${name} PRIVATE ${LLVM_PTHREAD_LIB})