[Lex] Make MacroDirective::findDirectiveAtLoc take const SourceManager

I'm currently working on including macro expansions in the Static Analyzer's
plist output, where I can only access a const SourceManager.

Differential Revision: https://reviews.llvm.org/D53940

llvm-svn: 345741
diff --git a/clang/lib/Lex/MacroInfo.cpp b/clang/lib/Lex/MacroInfo.cpp
index 4ed69ec..434c120 100644
--- a/clang/lib/Lex/MacroInfo.cpp
+++ b/clang/lib/Lex/MacroInfo.cpp
@@ -200,7 +200,8 @@
 }
 
 const MacroDirective::DefInfo
-MacroDirective::findDirectiveAtLoc(SourceLocation L, SourceManager &SM) const {
+MacroDirective::findDirectiveAtLoc(SourceLocation L,
+                                   const SourceManager &SM) const {
   assert(L.isValid() && "SourceLocation is invalid.");
   for (DefInfo Def = getDefinition(); Def; Def = Def.getPreviousDefinition()) {
     if (Def.getLocation().isInvalid() ||  // For macros defined on the command line.