Apply another batch of fixes from clang-tidy's performance-unnecessary-value-param.

Contains some manual fixes. No functionality change intended.

llvm-svn: 273047
diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index d0b2667..e22f4cb 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -679,9 +679,9 @@
 
 /// Parse the summary index out of an IR file and return the summary
 /// index object if found, or nullptr if not.
-static std::unique_ptr<ModuleSummaryIndex>
-getModuleSummaryIndexForFile(StringRef Path, std::string &Error,
-                             DiagnosticHandlerFunction DiagnosticHandler) {
+static std::unique_ptr<ModuleSummaryIndex> getModuleSummaryIndexForFile(
+    StringRef Path, std::string &Error,
+    const DiagnosticHandlerFunction &DiagnosticHandler) {
   std::unique_ptr<MemoryBuffer> Buffer;
   ErrorOr<std::unique_ptr<MemoryBuffer>> BufferOrErr =
       MemoryBuffer::getFile(Path);