Add -linker-path frontend option to choose the linker

In most Linux distributions, installing a clang package other than the
default unversioned one will not install a symlink from /usr/bin/clang++
-> /usr/bin/clang++-N-M, which can break builds with a not so great
diagnostic (a separate problem).

"ld" and "clang++" are hard-coded in the link job actions, so provide a
frontend flag, -linker-path, as a customization point for these.

rdar://problem/23537079

include/swift/Option/Options.td

@@ -408,6 +408,11 @@ def resource_dir : Separate<["-"], "resource-dir">,
   MetaVarName<"</usr/lib/swift>">,
   HelpText<"The directory that holds the compiler resource files">;
 
+def linker_path : Separate<["-"], "linker-path">,
+  Flags<[FrontendOption, HelpHidden]>,
+  MetaVarName<"<arg>">,
+  HelpText<"The path to the linker or C++ driver to use for linking">;
+
 def target : Separate<["-"], "target">,
   Flags<[FrontendOption, ModuleWrapOption]>,
   HelpText<"Generate code for the given target">;

lib/Driver/ToolChains.cpp

@@ -929,7 +929,10 @@ toolchains::Darwin::constructInvocation(const LinkJobAction &job,
   Arguments.push_back("-o");
   Arguments.push_back(context.Output.getPrimaryOutputFilename().c_str());
 
-  return std::make_pair("ld", Arguments);
+  auto Linker = "ld";
+  if (auto *Arg = context.Args.getLastArg(options::OPT_linker_path))
+    Linker = Arg->getValue();
+  return std::make_pair(Linker, Arguments);
 }
 
 #if defined(SWIFT_ENABLE_TARGET_LINUX)
@@ -1043,7 +1046,10 @@ toolchains::Linux::constructInvocation(const LinkJobAction &job,
   Arguments.push_back("-o");
   Arguments.push_back(context.Output.getPrimaryOutputFilename().c_str());
 
-  return std::make_pair("clang++", Arguments);
+  auto Linker = "clang++";
+  if (auto *Arg = context.Args.getLastArg(options::OPT_linker_path))
+    Linker = Arg->getValue();
+  return std::make_pair(Linker, Arguments);
 }
 
 #endif // SWIFT_ENABLE_TARGET_LINUX

test/Driver/linker.swift

@@ -173,6 +173,21 @@
 // RELATIVE_ARCLITE: /DISTINCTIVE-PATH/usr/lib/arc/libarclite_macosx.a
 // RELATIVE_ARCLITE: -o {{[^ ]+}}
 
+// RUN: %t/DISTINCTIVE-PATH/usr/bin/swiftc -target x86_64-apple-macosx10.9 %s -### | FileCheck -check-prefix=APPLE-LINKER %s
+// APPLE-LINKER: /usr/bin/ld
+// APPLE-LINKER-NOT: /path/to/my/linker
+
+// RUN: %t/DISTINCTIVE-PATH/usr/bin/swiftc -linker-path /path/to/my/linker -target x86_64-apple-macosx10.9 %s -### | FileCheck -check-prefix=APPLE-LINKER-OVERRIDE %s
+// APPLE-LINKER-OVERRIDE: /path/to/my/linker
+// APPLE-LINKER-OVERRIDE-NOT: /usr/bin/ld
+
+// RUN: %t/DISTINCTIVE-PATH/usr/bin/swiftc -target x86_64-unknown-linux-gnu %s -### | FileCheck -check-prefix=LINUX-LINKER %s
+// LINUX-LINKER: clang++
+// LINUX-LINKER-NOT: /path/to/my/linker
+
+// RUN: %t/DISTINCTIVE-PATH/usr/bin/swiftc -linker-path /path/to/my/linker -target x86_64-unknown-linux-gnu %s -### | FileCheck -check-prefix=LINUX-LINKER-OVERRIDE %s
+// LINUX-LINKER-OVERRIDE: /path/to/my/linker
+// LINUX-LINKER-OVERRIDE-NOT: clang++
 
 // Clean up the test executable because hard links are expensive.
 // RUN: rm -rf %t/DISTINCTIVE-PATH/usr/bin/swiftc