Fix uses of reserved identifiers starting with an underscore followed by an uppercase letter

This covers essentially all of llvm's headers and libs. One or two weird
cases I wasn't sure were worth/appropriate to fix.

llvm-svn: 232394
diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 4d0ba81..ea3dd45 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -185,9 +185,9 @@
     }
 
   public:
-    ELFObjectWriter(MCELFObjectTargetWriter *MOTW, raw_ostream &_OS,
+    ELFObjectWriter(MCELFObjectTargetWriter *MOTW, raw_ostream &OS,
                     bool IsLittleEndian)
-        : MCObjectWriter(_OS, IsLittleEndian), FWriter(IsLittleEndian),
+        : MCObjectWriter(OS, IsLittleEndian), FWriter(IsLittleEndian),
           TargetObjectWriter(MOTW), NeedsGOT(false) {}
 
     virtual ~ELFObjectWriter();