ANDROID: add support for ThinLTO

This change adds support for ThinLTO, which greatly improves build times
over full LTO while retaining most of the performance benefits:

  https://clang.llvm.org/docs/ThinLTO.html

Bug: 145210207
Change-Id: I8bfc19028266077be2bc1fb5c2bc001b599d3214
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Makefile

@@ -813,10 +813,16 @@ LDFLAGS_vmlinux += --gc-sections
 endif
 
 ifdef CONFIG_LTO_CLANG
-lto-clang-flags	:= -flto -fvisibility=hidden
+ifdef CONFIG_THINLTO
+lto-clang-flags	:= -flto=thin
+KBUILD_LDFLAGS	+= --thinlto-cache-dir=.thinlto-cache
+else
+lto-clang-flags	:= -flto
+endif
+lto-clang-flags += -fvisibility=default $(call cc-option, -fsplit-lto-unit)
 
 # allow disabling only clang LTO where needed
-DISABLE_LTO_CLANG := -fno-lto -fvisibility=default
+DISABLE_LTO_CLANG := -fno-lto
 export DISABLE_LTO_CLANG
 endif
 
@@ -829,7 +835,7 @@ export LTO_CFLAGS DISABLE_LTO
 endif
 
 ifdef CONFIG_CFI_CLANG
-cfi-clang-flags	+= -fsanitize=cfi $(call cc-option, -fsplit-lto-unit)
+cfi-clang-flags	+= -fsanitize=cfi
 DISABLE_CFI_CLANG := -fno-sanitize=cfi
 ifdef CONFIG_MODULES
 cfi-clang-flags	+= -fsanitize-cfi-cross-dso

arch/Kconfig

@@ -486,6 +486,18 @@ config ARCH_SUPPORTS_LTO_CLANG
 	  - compiling inline assembly with clang's integrated assembler,
 	  - and linking with LLD.
 
+config ARCH_SUPPORTS_THINLTO
+	bool
+	help
+	  An architecture should select this if it supports clang's ThinLTO.
+
+config THINLTO
+	bool "Use clang ThinLTO (EXPERIMENTAL)"
+	depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO
+	default y
+	help
+	  Use ThinLTO to speed up Link Time Optimization.
+
 choice
 	prompt "Link-Time Optimization (LTO) (EXPERIMENTAL)"
 	default LTO_NONE