[gn] Make -no-exceptions flag a config

[phosek]

Summary

This allows it to be disabled for targets that need exceptions
like libunwind, libc++abi and libc++.

[thakis]

llvm/trunk/utils/gn/build/BUILD.gn

67

This broke things on Windows. Do you know if /EHSs-c- should be in no_exceptions on Windows too? For now I'll just put fno-exceptions in a host_os != "win" block like it used to be, but maybe that isn't quite the right fix.

llvm/trunk/utils/gn/build/BUILD.gn

if (is_debug) {
       cflags += [ "-g" ]
     }
     if (is_optimized) {
       cflags += [ "-O3" ]
     }
     cflags += [ "-fdiagnostics-color" ]
     cflags_cc = [
       "-std=c++11",
-      "-fno-exceptions",
       "-fvisibility-inlines-hidden",
     ]
   } else {
     if (is_debug) {
       cflags += [
         "/Zi",
         "/FS",
       ]
defines += [
       "_CRT_NONSTDC_NO_WARNINGS",
       "_SCL_SECURE_NO_DEPRECATE",
       "_SCL_SECURE_NO_WARNINGS",
 
       "_HAS_EXCEPTIONS=0",
       "_UNICODE",
       "UNICODE",
     ]
     cflags += [ "/EHs-c-" ]
 
     # The MSVC default value (1 MB) is not enough for parsing recursive C++
     # templates in Clang.
     ldflags += [ "/STACK:10000000" ]
   }
 
   # Warning setup.
   if (host_os == "win" && !is_clang) {
config("compiler_defaults") {
   }
 
   # On Windows, the linker is not invoked through the compiler driver.
   if (use_lld && host_os != "win") {
     ldflags += [ "-fuse-ld=lld" ]
   }
 }
 
+config("no_exceptions") {
+  cflags_cc = [ "-fno-exceptions" ]
+}
+
 config("no_rtti") {
   if (current_os == "win") {
     cflags_cc = [ "/GR-" ]
   } else {
     cflags_cc = [ "-fno-rtti" ]
   }
 }
 

llvm/trunk/utils/gn/build/BUILDCONFIG.gn

 # All targets will get this list of configs by default.
 # Targets can opt out of a config by removing it from their local configs list.
 # If you're adding global flags and don't need targets to be able to opt out,
 # add the flags to compiler_defaults, not to a new config.
 shared_binary_target_configs = [
   "//llvm/utils/gn/build:compiler_defaults",
   "//llvm/utils/gn/build:llvm_code",
+  "//llvm/utils/gn/build:no_exceptions",
   "//llvm/utils/gn/build:no_rtti",
   "//llvm/utils/gn/build:thin_archive",
   "//llvm/utils/gn/build:warn_covered_switch_default",
 ]
 
 # Apply that default list to the binary target types.
 set_defaults("executable") {
   configs = shared_binary_target_configs