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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166183 91177308-0d34-0410-b5e6-96231b3b80d8