Reduce the number of implicit StringRef->std::string conversions by threading StringRef through more APIs.

No functionality change intended.

llvm-svn: 260815
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index ac78da2..82bc92c 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3483,7 +3483,7 @@
   }
 }
 
-ASTReader::ASTReadResult ASTReader::ReadAST(const std::string &FileName,
+ASTReader::ASTReadResult ASTReader::ReadAST(StringRef FileName,
                                             ModuleKind Type,
                                             SourceLocation ImportLoc,
                                             unsigned ClientLoadCapabilities) {