[ms-inline asm] Use parsePrimaryExpr in lieu of parseExpression if we need to
parse an identifier.  Otherwise, parseExpression may parse multiple tokens,
which makes it impossible to properly compute an immediate displacement.
An example of such a case is the source operand (i.e., [Symbol + ImmDisp]) in
the below example:

 __asm mov eax, [Symbol + ImmDisp]

The existing test cases exercise this patch.
rdar://13611297