MC: permit emitting a symbol value as section relative

This adds an optional parameter to the EmitSymbolValue method in MCStreamer to
permit emitting a symbol value as a section relative value.  This is to cover
the use in MCDwarf which should not really know about how to emit a section
relative value for a given target.

This addresses post-review comments from Eric Christopher in SVN r213275.

llvm-svn: 213463
diff --git a/llvm/lib/MC/MCStreamer.cpp b/llvm/lib/MC/MCStreamer.cpp
index f356402..46e80cc 100644
--- a/llvm/lib/MC/MCStreamer.cpp
+++ b/llvm/lib/MC/MCStreamer.cpp
@@ -148,8 +148,15 @@
   EmitValueImpl(Value, Size, Loc);
 }
 
-void MCStreamer::EmitSymbolValue(const MCSymbol *Sym, unsigned Size) {
-  EmitValueImpl(MCSymbolRefExpr::Create(Sym, getContext()), Size);
+void MCStreamer::EmitSymbolValue(const MCSymbol *Sym, unsigned Size,
+                                 bool IsSectionRelative) {
+  assert((!IsSectionRelative || Size == 4) &&
+         "SectionRelative value requires 4-bytes");
+
+  if (!IsSectionRelative)
+    EmitValueImpl(MCSymbolRefExpr::Create(Sym, getContext()), Size);
+  else
+    EmitCOFFSecRel32(Sym);
 }
 
 void MCStreamer::EmitGPRel64Value(const MCExpr *Value) {