Change Lexer::MeasureTokenLength to take a LangOptions reference.

This allows it to accurately measure tokens, so that we get:

t.cpp:8:13: error: unknown type name 'X'
static foo::X  P;
       ~~~~~^

instead of the woefully inferior:

t.cpp:8:13: error: unknown type name 'X'
static foo::X  P;
       ~~~~ ^

Most of this is just plumbing to push the reference around.

llvm-svn: 69099