[ms] [llvm-ml] Make variable redefinition match ML.EXE

MASM specifies that all variable definitions are redefinable, except for EQU definitions to expressions. (TEXTEQU is unspecified, but appears to be fully redefinable as well.)

Also, in practice, ML.EXE allows redefinitions where the value doesn't change.

Make variable redefinition possible for text macros, suppressing expansion if written as the first argument to an EQU or TEXTEQU directive.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D103993