Frontend: add -autolink-library= option to support Windows

All modules on Windows need to link against one of {libcmtd.lib, libcmt.lib,
msvcrtd.lib, msvcrt.lib}.  In addition to being the C library, it is the
equivalent of crtbegin0.o on other targets.  It is responsible for providing the
entry point itself.  Traditionally, cl will embed the linkage requirement into
all objects based on the flags given -- one of {/MTd, /MT, /MDd, /MD}.  clang
emulates this via the `--dependent-lib=` option.  Emulate that behaviour in the
swift driver so that swift objects being compiled for Windows targets can
auto-link to the required libraries.

Author: compnerd

include/swift/Option/Options.td

@@ -272,6 +272,9 @@ def Xlinker : Separate<["-"], "Xlinker">,
   Flags<[DoesNotAffectIncrementalBuild]>,
   HelpText<"Specifies an option which should be passed to the linker">;
 
+def autolink_library : Joined<["-"], "autolink-library=">,
+  HelpText<"Add dependent library">, Flags<[FrontendOption]>;
+
 // Optimization levels
 
 def O_Group : OptionGroup<"<optimization level options>">;

lib/Frontend/CompilerInvocation.cpp

@@ -1285,6 +1285,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.EnableReflectionNames = false;
   }
 
+  for (const auto &Lib : Args.getAllArgValues(options::OPT_autolink_library))
+    Opts.LinkLibraries.push_back(LinkLibrary(Lib, LibraryKind::Library));
+
   return false;
 }
 

test/IRGen/dependent-library.swift

@@ -0,0 +1,7 @@
+// RUN: %swift -target i686-unknown-windows-msvc -emit-ir -parse-as-library -parse-stdlib -module-name dependent -autolink-library=oldnames -autolink-library=msvcrt %s -o - | FileCheck %s
+
+// CHECK: !{i32 6, !"Linker Options", ![[options:[0-9]+]]}
+// CHECK: ![[options]] = !{![[oldnames:[0-9]+]], ![[msvcrtd:[0-9]+]]}
+// CHECK: ![[oldnames]] = !{!"/DEFAULTLIB:oldnames.lib"}
+// CHECK: ![[msvcrtd]] = !{!"/DEFAULTLIB:msvcrt.lib"}
+