Refactored duplicate string literal lexing code within Preprocessor, into a

common LexStringLiteral function.  In doing so, some consistency problems have
been ironed out (e.g. where the first token in the string literal was lexed
with macro expansion, but subsequent ones were not) and also an erroneous
diagnostic has been corrected.

LexStringLiteral is complemented by a FinishLexStringLiteral function which
can be used in the situation where the first token of the string literal has
already been lexed.

llvm-svn: 168266