Make ParseIdentifier a public method instead of private.

The MCAsmParser interface defines ParseIdentifier is public. There's no reason
whatsoever for AsmParser (which implements the MCAsmParser interface) to hide
this method.

This is all part of a bigger scheme. Several asm parsing "extensions" use the
main parser properly through the MCAsmParser interface. However,
GenericAsmParser has much more exclusive access and uses implementation details
from the concrete implementation - AsmParser, in which it is also declared as
a friend. This makes for overly coupled code, and even makes it hard to split
GenericAsmParser into a separate file. There's no reason why GenericAsmParser
shouldn't be able to access AsmParser through an abstract interface, as long
as it's actually registered as an extension.