[ms] [llvm-ml] Disambiguate size directives and variable declarations

MASM allows statements of the form:
	<VAR> DWORD 5
to declare a variable with name <VAR>, while:
	call dword ptr [<value>]
is a valid instruction. To disambiguate, we recognize size directives by the trailing "ptr" token.

As discussed in https://lists.llvm.org/pipermail/llvm-dev/2021-May/150774.html

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D103257