SourceManager: Use setMainFileID() consistently

Eliminate createMainFileID() / createMainFileIDForMemBuffer() utility
functions. These didn't add much convenience and conflated two distinct
operations.

This change makes things easier to follow by providing a consistent interface
and getting rid of a bunch of cast-to-voids.

llvm-svn: 209266
diff --git a/clang/unittests/Lex/LexerTest.cpp b/clang/unittests/Lex/LexerTest.cpp
index 7be9360..3d30352 100644
--- a/clang/unittests/Lex/LexerTest.cpp
+++ b/clang/unittests/Lex/LexerTest.cpp
@@ -64,7 +64,7 @@
   std::vector<Token> CheckLex(StringRef Source,
                               ArrayRef<tok::TokenKind> ExpectedTokens) {
     MemoryBuffer *buf = MemoryBuffer::getMemBuffer(Source);
-    (void) SourceMgr.createMainFileIDForMemBuffer(buf);
+    SourceMgr.setMainFileID(SourceMgr.createFileID(buf));
 
     VoidModuleLoader ModLoader;
     HeaderSearch HeaderInfo(new HeaderSearchOptions, SourceMgr, Diags, LangOpts,