[Lexer] Allow UCN for dollar symbol '\u0024' in identifiers when using -fdollars...

[Lexer] Allow UCN for dollar symbol '\u0024' in identifiers when using -fdollars-in-identifiers flag.

Summary:
Previously, the -fdollars-in-identifiers flag allows the '$' symbol to be used
in an identifier but the universal character name equivalent '\u0024' is not
allowed.
This patch changes this, so that \u0024 is valid in identifiers.

Reviewers: rsmith, jordan_rose

Reviewed By: rsmith

Subscribers: dexonsmith, simoncook, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71758