CodeGen, Target: Move Mach-O-specific symbol name logic to Mach-O lowering.

A profile of an LTO link of Chrome revealed that we were spending some
~30-50% of execution time in the function Constant::getRelocationInfo(),
which is called from TargetLoweringObjectFile::getKindForGlobal() and in turn
from TargetMachine::getNameWithPrefix().

It turns out that we only need the result of getKindForGlobal() when
targeting Mach-O, so this change moves the relevant part of the logic to
TargetLoweringObjectFileMachO.

NFCI.

Differential Revision: http://reviews.llvm.org/D14168

llvm-svn: 252014
diff --git a/llvm/lib/Target/TargetLoweringObjectFile.cpp b/llvm/lib/Target/TargetLoweringObjectFile.cpp
index 856d92d..dd65b88 100644
--- a/llvm/lib/Target/TargetLoweringObjectFile.cpp
+++ b/llvm/lib/Target/TargetLoweringObjectFile.cpp
@@ -344,7 +344,7 @@
 }
 
 void TargetLoweringObjectFile::getNameWithPrefix(
-    SmallVectorImpl<char> &OutName, const GlobalValue *GV,
-    bool CannotUsePrivateLabel, Mangler &Mang, const TargetMachine &TM) const {
-  Mang.getNameWithPrefix(OutName, GV, CannotUsePrivateLabel);
+    SmallVectorImpl<char> &OutName, const GlobalValue *GV, Mangler &Mang,
+    const TargetMachine &TM) const {
+  Mang.getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false);
 }