Apply most suggestions of clang-tidy's performance-unnecessary-value-param

Avoids unnecessary copies. All changes audited & pass tests with asan.
No functional change intended.

llvm-svn: 272190
diff --git a/llvm/lib/Target/NVPTX/NVPTXUtilities.cpp b/llvm/lib/Target/NVPTX/NVPTXUtilities.cpp
index 578b466..835e4b4 100644
--- a/llvm/lib/Target/NVPTX/NVPTXUtilities.cpp
+++ b/llvm/lib/Target/NVPTX/NVPTXUtilities.cpp
@@ -99,7 +99,7 @@
   }
 }
 
-bool llvm::findOneNVVMAnnotation(const GlobalValue *gv, std::string prop,
+bool llvm::findOneNVVMAnnotation(const GlobalValue *gv, const std::string &prop,
                                  unsigned &retval) {
   MutexGuard Guard(Lock);
   const Module *m = gv->getParent();
@@ -113,7 +113,7 @@
   return true;
 }
 
-bool llvm::findAllNVVMAnnotation(const GlobalValue *gv, std::string prop,
+bool llvm::findAllNVVMAnnotation(const GlobalValue *gv, const std::string &prop,
                                  std::vector<unsigned> &retval) {
   MutexGuard Guard(Lock);
   const Module *m = gv->getParent();