[CMake][Fuchsia] Define asan+noexcept multilib

[phosek]

Summary

Using noexcept multilib with -fno-exceptions can lead to significant
space savings when statically linking libc++abi because we don't need
all the unwinding and demangling code.

When compiling with ASan, we already get a lot of overhead from the
instrumentation itself, when statically linking libc++abi, that overhead
is even larger.

Having the noexcept variant for ASan can help significantly, we've seen
more than 50% size reduction in our system image, which offsets the cost
of having to build another multilib.

[mcgrathr]

lgtm

clang/cmake/caches/Fuchsia-stage2.cmake

foreach(target x86_64;aarch64)
     set(RUNTIMES_${target}-unknown-fuchsia+asan_LLVM_USE_SANITIZER "Address" CACHE STRING "")
     set(RUNTIMES_${target}-unknown-fuchsia+asan_LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS OFF CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia+asan_LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS OFF CACHE BOOL "")
 
     set(RUNTIMES_${target}-unknown-fuchsia+noexcept_LLVM_BUILD_COMPILER_RT OFF CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia+noexcept_LIBCXXABI_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
     set(RUNTIMES_${target}-unknown-fuchsia+noexcept_LIBCXX_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
 
+    set(RUNTIMES_${target}-unknown-fuchsia+asan+noexcept_LLVM_BUILD_COMPILER_RT OFF CACHE BOOL "")
+    set(RUNTIMES_${target}-unknown-fuchsia+asan+noexcept_LLVM_USE_SANITIZER "Address" CACHE STRING "")
+    set(RUNTIMES_${target}-unknown-fuchsia+asan+noexcept_LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS OFF CACHE BOOL "")
+    set(RUNTIMES_${target}-unknown-fuchsia+asan+noexcept_LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS OFF CACHE BOOL "")
+    set(RUNTIMES_${target}-unknown-fuchsia+asan+noexcept_LIBCXXABI_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
+    set(RUNTIMES_${target}-unknown-fuchsia+asan+noexcept_LIBCXX_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
+
     # Use .build-id link.
     list(APPEND RUNTIME_BUILD_ID_LINK "${target}-unknown-fuchsia")
   endforeach()
 
-  set(LLVM_RUNTIME_MULTILIBS "asan;noexcept" CACHE STRING "")
+  set(LLVM_RUNTIME_MULTILIBS "asan;noexcept;asan+noexcept" CACHE STRING "")
   set(LLVM_RUNTIME_MULTILIB_asan_TARGETS "x86_64-unknown-fuchsia;aarch64-unknown-fuchsia" CACHE STRING "")
   set(LLVM_RUNTIME_MULTILIB_noexcept_TARGETS "x86_64-unknown-fuchsia;aarch64-unknown-fuchsia" CACHE STRING "")
+  set(LLVM_RUNTIME_MULTILIB_asan+noexcept_TARGETS "x86_64-unknown-fuchsia;aarch64-unknown-fuchsia" CACHE STRING "")
 endif()
 
 set(LLVM_BUILTIN_TARGETS "${BUILTIN_TARGETS}" CACHE STRING "")
 set(LLVM_RUNTIME_TARGETS "${RUNTIME_TARGETS}" CACHE STRING "")
 set(LLVM_RUNTIME_BUILD_ID_LINK_TARGETS "${RUNTIME_BUILD_ID_LINK}" CACHE STRING "")
 
 # Setup toolchain.
 set(LLVM_INSTALL_TOOLCHAIN_ONLY ON CACHE BOOL "")

clang/lib/Driver/ToolChains/Fuchsia.cpp

Fuchsia::Fuchsia(const Driver &D, const llvm::Triple &Triple,
   Multilibs.push_back(Multilib());
   // Use the noexcept variant with -fno-exceptions to avoid the extra overhead.
   Multilibs.push_back(Multilib("noexcept", {}, {}, 1)
                           .flag("-fexceptions")
                           .flag("+fno-exceptions"));
   // ASan has higher priority because we always want the instrumentated version.
   Multilibs.push_back(Multilib("asan", {}, {}, 2)
                           .flag("+fsanitize=address"));
+  // Use the asan+noexcept variant with ASan and -fno-exceptions.
+  Multilibs.push_back(Multilib("asan+noexcept", {}, {}, 3)
+                          .flag("+fsanitize=address")
+                          .flag("-fexceptions")
+                          .flag("+fno-exceptions"));
   Multilibs.FilterOut([&](const Multilib &M) {
     std::vector<std::string> RD = FilePaths(M);
     return std::all_of(RD.begin(), RD.end(), [&](std::string P) {
       return !getVFS().exists(P);
     });
   });
 
   Multilib::flags_list Flags;