[ms-inline asm] Add support for the minus unary operator.  Previously, we were
unable to handle cases such as __asm mov eax, 8*-8.

This patch also attempts to simplify the state machine.  Further, the error
reporting has been improved.  Test cases included, but more will be added to
the clang side shortly.
rdar://13668445


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179719 91177308-0d34-0410-b5e6-96231b3b80d8