[FatLTO] Allow -fno-fat-lto-objects to override -ffat-lto-objects (#128157)

For builds that cannot be easily modified and enabled with
`-ffat-lto-objects`, `-fno-fat-lto-objects` acts as an escape hatch to
disable this option (which is standard to how clang and lld flags are
used).

Author: thevinster

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -864,7 +864,8 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
   const bool IsAMDGCN = Triple.isAMDGCN();
   const char *Linker = Args.MakeArgString(ToolChain.GetLinkerPath());
   const Driver &D = ToolChain.getDriver();
-  const bool IsFatLTO = Args.hasArg(options::OPT_ffat_lto_objects);
+  const bool IsFatLTO = Args.hasFlag(options::OPT_ffat_lto_objects,
+                                     options::OPT_fno_fat_lto_objects, false);
   const bool IsUnifiedLTO = Args.hasArg(options::OPT_funified_lto);
   if (llvm::sys::path::filename(Linker) != "ld.lld" &&
       llvm::sys::path::stem(Linker) != "ld.lld" && !Triple.isOSOpenBSD()) {

clang/test/Driver/fat-lto-objects.c

@@ -41,6 +41,8 @@
 // CHECK-CC-NOLTO-SAME: -emit-obj
 // CHECK-CC-NOLTO-NOT: -ffat-lto-objects
 
+// RUN: %clang --target=x86_64-unknown-linux-gnu -flto -ffat-lto-objects -fno-fat-lto-objects -### %s -c 2>&1 | FileCheck %s -check-prefix=CHECK-CC-NOLTO
+
 /// We need to pass an additional flag (--fat-lto-objects) to lld when linking w/ -flto -ffat-lto-objects
 /// But it should not be there when LTO is disabled w/ -fno-lto
 // RUN: %clang --target=x86_64-unknown-linux-gnu --sysroot=%S/Inputs/basic_cross_linux_tree %s \