Remove some 'const' specifiers that do nothing but prevent moving the argument.

Found by clang-tidy's misc-move-const-arg. While there drop some
obsolete c_str() calls.

llvm-svn: 271181
diff --git a/llvm/lib/IR/Function.cpp b/llvm/lib/IR/Function.cpp
index 36800ee..c874196 100644
--- a/llvm/lib/IR/Function.cpp
+++ b/llvm/lib/IR/Function.cpp
@@ -405,7 +405,7 @@
   return getContext().getGC(*this);
 }
 
-void Function::setGC(const std::string Str) {
+void Function::setGC(std::string Str) {
   setValueSubclassDataBit(14, !Str.empty());
   getContext().setGC(*this, std::move(Str));
 }