[llvm-ml] Disallow '{' and '}' at the start of a statement

'{', '}' and (conditionally) '*' were allowed at the start of a
statement. This behavior was copied from AsmParser, where they were
added to support Hexagon bundles (braces) and BFP memory instructions
(the star).
MASM dialect is x86-specific and does not allow these symbols to be
used at the beginning of an instruction.

Worth noting that '{' is a valid first character in AT&T syntax
(e.g. "{vex} vpmadd52huq ..."); MASM variant is to omit the braces.

Reviewed By: epastor

Differential Revision: https://reviews.llvm.org/D142321