MC tweak symbol difference resolution for non-local symbols.

When the non-local symbol in the expression is in the same fragment
as the second symbol, the assembler can still evaluate the expression
without needing a relocation.

For example, on ARM:
_foo:
	ldr lr, (_foo - 4)

rdar://10348687



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148341 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/MC/MachObjectWriter.cpp b/lib/MC/MachObjectWriter.cpp
index e016f09..ce222b7 100644
--- a/lib/MC/MachObjectWriter.cpp
+++ b/lib/MC/MachObjectWriter.cpp
@@ -590,8 +590,8 @@
     // assembler locals.
 
     if (!Asm.getBackend().hasReliableSymbolDifference()) {
-      if ((!SA.isTemporary() && Asm.getSubsectionsViaSymbols()) ||
-           !SA.isInSection() || &SecA != &SecB)
+      if (!SA.isInSection() || &SecA != &SecB ||
+          (!SA.isTemporary() && &FB != Asm.getSymbolData(SA).getFragment()))
         return false;
       return true;
     }