Reapply r179115, but use parsePrimaryExpression a little more judiciously.
Test cases that regressed due to r179115, plus a few more, were added in
r179182.  Original commit message below:

[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]

Part of rdar://13611297