Lex: Don't restrict legal UCNs when preprocessing assembly

The C and C++ standards disallow using universal character names to
refer to some characters, such as basic ascii and control characters,
so we reject these sequences in the lexer. However, when the
preprocessor isn't being used on C or C++, it doesn't make sense to
apply these restrictions.

Notably, accepting these characters avoids issues with unicode escapes
when GHC uses the compiler as a preprocessor on haskell sources.

Fixes rdar://problem/14742289

llvm-svn: 193067