[Parser] Misparsed function parameter with keyword argument '_const'

[Kai Lau]

Description

One would expect FunctionParameterSyntax.firstName to be "_const",

func const(_const map: String) {}

However the parser incorrectly recognizes _const as DeclModifier, and FunctionParameterSyntax.firstName becomes "map".

Steps to Reproduce

No response

[Alex Hoppen]

Synced to Apple’s issue tracker as rdar://133105836