Refactor and cleanup Assembly Parsing / Lexing

Add explicit Comment Token in Assembly Lexing for future support for
outputting explicit comments from inline assembly. As part of this,
CPPHash Directives are now explicitly distinguished from Hash line
comments in Lexer.

Line comments are recorded as EndOfStatement tokens, not Comment tokens
to simplify compatibility with current TargetParsers. This slightly
complicates comment output.

This remove all lexing tasks out of the parser, does minor cleanup
to remove extraneous newlines Asm Output, and some improvements white
space handling.

Reviewers: rtrieu, dwmw2, rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20009

llvm-svn: 272953