[MC] Teach the MachO object writer about N_FUNC_COLD

N_FUNC_COLD is a new MachO symbol attribute. It's a hint to the linker
to order a symbol towards the end of its section, to improve locality.

Example:

```
void a1() {}
__attribute__((cold)) void a2() {}
void a3() {}
int main() {
  a1();
  a2();
  a3();
  return 0;
}
```

A linker that supports N_FUNC_COLD will order _a2 to the end of the text
section. From `nm -njU` output, we see:

```
_a1
_a3
_main
_a2
```

Differential Revision: https://reviews.llvm.org/D57190

llvm-svn: 352227

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCParser/AsmParser.cpp b/llvm/lib/MC/MCParser/AsmParser.cpp
index 6c311c13836..189422e2ad3 100644
--- a/llvm/lib/MC/MCParser/AsmParser.cpp
+++ b/llvm/lib/MC/MCParser/AsmParser.cpp
@@ -426,6 +426,7 @@ private:
     DK_WEAK_DEFINITION,
     DK_WEAK_REFERENCE,
     DK_WEAK_DEF_CAN_BE_HIDDEN,
+    DK_COLD,
     DK_COMM,
     DK_COMMON,
     DK_LCOMM,
@@ -1983,6 +1984,8 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
       return parseDirectiveSymbolAttribute(MCSA_WeakReference);
     case DK_WEAK_DEF_CAN_BE_HIDDEN:
       return parseDirectiveSymbolAttribute(MCSA_WeakDefAutoPrivate);
+    case DK_COLD:
+      return parseDirectiveSymbolAttribute(MCSA_Cold);
     case DK_COMM:
     case DK_COMMON:
       return parseDirectiveComm(/*IsLocal=*/false);
@@ -5223,6 +5226,7 @@ void AsmParser::initializeDirectiveKindMap() {
   DirectiveKindMap[".weak_definition"] = DK_WEAK_DEFINITION;
   DirectiveKindMap[".weak_reference"] = DK_WEAK_REFERENCE;
   DirectiveKindMap[".weak_def_can_be_hidden"] = DK_WEAK_DEF_CAN_BE_HIDDEN;
+  DirectiveKindMap[".cold"] = DK_COLD;
   DirectiveKindMap[".comm"] = DK_COMM;
   DirectiveKindMap[".common"] = DK_COMMON;
   DirectiveKindMap[".lcomm"] = DK_LCOMM;