[clang] Make --ld-path= work with -fuse-ld=lld

[thakis]

Summary

This allows using --ld-path= to set a custom linker path, while
still informing clang that the binary at that path is an lld built
at the same revision as clang, so that clang can make assumptions
about the flags it supports, its output format, etc.

This currently only has an observable effect on Darwin.

[MaskRay]

Looks great!

clang/lib/Driver/ToolChain.cpp

std::string ToolChain::GetLinkerPath(bool *LinkerIsLLD) const {
   // Get -fuse-ld= first to prevent -Wunused-command-line-argument. -fuse-ld= is
   // considered as the linker flavor, e.g. "bfd", "gold", or "lld".
   const Arg* A = Args.getLastArg(options::OPT_fuse_ld_EQ);
   StringRef UseLinker = A ? A->getValue() : CLANG_DEFAULT_LINKER;
 
   // --ld-path= takes precedence over -fuse-ld= and specifies the executable
   // name. -B, COMPILER_PATH and PATH and consulted if the value does not
   // contain a path component separator.
+  // -fuse-ld=lld can be used with --ld-path= to inform clang that the binary
+  // that --ld-path= points to is lld.
   if (const Arg *A = Args.getLastArg(options::OPT_ld_path_EQ)) {
     std::string Path(A->getValue());
     if (!Path.empty()) {
       if (llvm::sys::path::parent_path(Path).empty())
         Path = GetProgramPath(A->getValue());
-      if (llvm::sys::fs::can_execute(Path))
+      if (llvm::sys::fs::can_execute(Path)) {
+        if (LinkerIsLLD)
+          *LinkerIsLLD = UseLinker == "lld";
         return std::string(Path);
       }
+    }
     getDriver().Diag(diag::err_drv_invalid_linker_name) << A->getAsString(Args);
     return GetProgramPath(getDefaultLinker());
   }
   // If we're passed -fuse-ld= with no argument, or with the argument ld,
   // then use whatever the default system linker is.
   if (UseLinker.empty() || UseLinker == "ld") {
     const char *DefaultLinker = getDefaultLinker();
     if (llvm::sys::path::is_absolute(DefaultLinker))

clang/test/Driver/darwin-ld-demangle-lld.c

 // With -fuse-ld=lld, -demangle is always passed to the linker on Darwin.
 // REQUIRES: shell
 
-// RUN: %clang --target=x86_64-apple-darwin -### \
-// RUN:   -fuse-ld=lld -B%S/Inputs/lld -mlinker-version=0 %s 2>&1 \
+// RUN: %clang --target=x86_64-apple-darwin -### -fuse-ld=lld \
+// RUN:   -B%S/Inputs/lld -mlinker-version=0 %s 2>&1 \
+// RUN:   | FileCheck %s
+
+// RUN: %clang --target=x86_64-apple-darwin -### -fuse-ld=lld \
+// RUN:   --ld-path=%S/Inputs/lld/ld64.lld -mlinker-version=0 %s 2>&1 \
 // RUN:   | FileCheck %s
 
 // CHECK: "-demangle"