[CommonOptionsParser] Return source paths as const ref.

This saves a superfluous copy and makes managing the lifetime of the
returned strings a bit easier.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271262 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Tooling/CommonOptionsParser.h

@@ -98,7 +98,7 @@ public:
   }
 
   /// Returns a list of source file paths to process.
-  std::vector<std::string> getSourcePathList() {
+  const std::vector<std::string> &getSourcePathList() const {
     return SourcePathList;
   }