Improve tracking of source position in the presence of /* */ comments.

Don't let the preprocessor remove comments anymore, so that the rules
in lex.ll can handle them.  Fix lCComment() to update the source
position as it eats characters in comments.

lex.ll

@@ -309,8 +309,10 @@ lParseBinary(const char *ptr, SourcePos pos, char **endPtr) {
 static void
 lCComment(SourcePos *pos) {
     char c, prev = 0;
-  
+
     while ((c = yyinput()) != 0) {
+        ++pos->last_column;
+
         if (c == '\n') {
             pos->last_line++;
             pos->last_column = 1;

module.cpp

@@ -1170,6 +1170,10 @@ Module::execPreprocessor(const char* infilename, llvm::raw_string_ostream* ostre
     inst.createSourceManager(inst.getFileManager());
     inst.InitializeSourceManager(infilename);
 
+    // Don't remove comments in the preprocessor, so that we can accurately
+    // track the source file position by handling them ourselves.
+    inst.getPreprocessorOutputOpts().ShowComments = 1;
+
     clang::PreprocessorOptions &opts = inst.getPreprocessorOpts();
 
     // Add defs for ISPC and PI