The global prefix is always one char. Don't use a string for it.

llvm-svn: 195926
diff --git a/llvm/lib/MC/MCAsmInfoCOFF.cpp b/llvm/lib/MC/MCAsmInfoCOFF.cpp
index 9d9f98e..54ae3a2 100644
--- a/llvm/lib/MC/MCAsmInfoCOFF.cpp
+++ b/llvm/lib/MC/MCAsmInfoCOFF.cpp
@@ -18,7 +18,7 @@
 void MCAsmInfoCOFF::anchor() { }
 
 MCAsmInfoCOFF::MCAsmInfoCOFF() {
-  GlobalPrefix = "_";
+  GlobalPrefix = '_';
   // MingW 4.5 and later support .comm with log2 alignment, but .lcomm uses byte
   // alignment.
   COMMDirectiveAlignmentIsInBytes = false;