toString function take a const refs where possible

Differential Revision: https://reviews.llvm.org/D40824

llvm-svn: 319787
diff --git a/lld/wasm/InputFiles.cpp b/lld/wasm/InputFiles.cpp
index 9cb4e08..bb93620 100644
--- a/lld/wasm/InputFiles.cpp
+++ b/lld/wasm/InputFiles.cpp
@@ -264,7 +264,7 @@
 }
 
 // Returns a string in the format of "foo.o" or "foo.a(bar.o)".
-std::string lld::toString(wasm::InputFile *File) {
+std::string lld::toString(const wasm::InputFile *File) {
   if (!File)
     return "<internal>";