[ms-inline asm] Move most of the AsmParsing logic in clang back into the MC

layer.  Add the ParseMSInlineAsm() function, which is the new interface to 
clang.  Also expose the new MCAsmParserSemaCallback interface, which is used
by the back-end to do name lookup in Sema.  Finally, remove the now defunct
APIs introduced in r165946.

llvm-svn: 166183