Spread some const around for non-mutating uses of MCSymbolData.

I discovered this const-hole while attempting to coalesnce the Symbol
and SymbolMap data structures. There's some pending issues with that,
but I figured this change was easy to flush early.

llvm-svn: 207124
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
index 1db517f..c16b355 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp
@@ -603,7 +603,7 @@
   // the offset when the destination has the same MCFragment.
   if (A && (unsigned)Fixup.getKind() == ARM::fixup_arm_thumb_bl) {
     const MCSymbol &Sym = A->getSymbol().AliasedSymbol();
-    MCSymbolData &SymData = Asm.getSymbolData(Sym);
+    const MCSymbolData &SymData = Asm.getSymbolData(Sym);
     IsResolved = (SymData.getFragment() == DF);
   }
   // We must always generate a relocation for BL/BLX instructions if we have