MCContext::reset: clear RelSecnames & MacroMap

When MCContext is used for the second compile of
llvm/test/MC/ELF/twice.ll, ensure that .rel.text and .rel.eh_frame
strings do not come from the previous compilation copies.

Author: MaskRay

llvm/lib/MC/MCContext.cpp

@@ -177,6 +177,8 @@ void MCContext::reset() {
   XCOFFUniquingMap.clear();
   DXCUniquingMap.clear();
 
+  RelSecNames.clear();
+  MacroMap.clear();
   ELFEntrySizeMap.clear();
   ELFSeenGenericMergeableSections.clear();