[CMAKE] Enable FatLTO as a build option for LLVM (#80480)

Since LLVM supports `-ffat-lto-objects` we should enable this as an
option in the LLVM build. FatLTO should improve the time it takes to
build tests for LTO enabled builds of the compiler by not linking w/ the
bitcode portion of the object files, which should speed up build times
for LTO builds without disabling optimizations.

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -11,6 +11,7 @@ set(LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "
 
 set(LLVM_ENABLE_BACKTRACES OFF CACHE BOOL "")
 set(LLVM_ENABLE_DIA_SDK OFF CACHE BOOL "")
+set(LLVM_ENABLE_FATLTO ON CACHE BOOL "")
 set(LLVM_ENABLE_HTTPLIB ON CACHE BOOL "")
 set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
 set(LLVM_ENABLE_LIBEDIT OFF CACHE BOOL "")

llvm/cmake/modules/AddLLVM.cmake

@@ -1637,8 +1637,14 @@ function(add_unittest test_suite test_name)
   # The runtime benefits of LTO don't outweight the compile time costs for tests.
   if(LLVM_ENABLE_LTO)
     if((UNIX OR MINGW) AND LINKER_IS_LLD)
-      set_property(TARGET ${test_name} APPEND_STRING PROPERTY
-                    LINK_FLAGS " -Wl,--lto-O0")
+      if(LLVM_ENABLE_FATLTO)
+        # When using FatLTO, just use relocatable linking.
+        set_property(TARGET ${test_name} APPEND_STRING PROPERTY
+                      LINK_FLAGS " -Wl,--no-fat-lto-objects")
+      else()
+        set_property(TARGET ${test_name} APPEND_STRING PROPERTY
+                      LINK_FLAGS " -Wl,--lto-O0")
+      endif()
     elseif(LINKER_IS_LLD_LINK)
       set_property(TARGET ${test_name} APPEND_STRING PROPERTY
                     LINK_FLAGS " /opt:lldlto=0")

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -32,6 +32,8 @@ endif()
 set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
 string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
 
+option(LLVM_ENABLE_FATLTO "Build LLVM with -ffat-lto-objects." OFF)
+
 # Ninja Job Pool support
 # The following only works with the Ninja generator in CMake >= 3.0.
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
@@ -1280,6 +1282,13 @@ elseif(LLVM_ENABLE_LTO)
   endif()
 endif()
 
+if(LLVM_ENABLE_FATLTO AND (FUCHSIA OR UNIX))
+  append("-ffat-lto-objects" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  if(NOT LINKER_IS_LLD_LINK)
+    append("-ffat-lto-objects" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS)
+  endif()
+endif()
+
 # Set an AIX default for LLVM_EXPORT_SYMBOLS_FOR_PLUGINS based on whether we are
 # doing dynamic linking (see below).
 set(LLVM_EXPORT_SYMBOLS_FOR_PLUGINS_AIX_default OFF)