[SR-615][Compiler] Don't error when lexing UTF-8 BOM

[tanadeau]

No description provided.

[lattner]

Thanks for tackling this, some comments:

• The patch will have the check be performed once per token lexed, it would make more sense for this to happen once per source file. As such, please move this to Lexer::Lexer or perhaps Lexer::primeLexer.
• Please include a test case.

Thanks!

[lattner]

FWIW, the analogous code in clang is here:
https://github.com/apple/swift-clang/blob/stable/lib/Lex/Lexer.cpp#L68

[tanadeau]

I moved the check to the Lexer's constructor and added a test case.

[lattner]

LGTM, thanks!