Patch for bug c++/17964 (cpp error locations for C++)

[Gabriel Dos Reis]

Mark Mitchell <mark@codesourcery.com> writes:

| Joseph S. Myers wrote:
| > This patch fixes bug 17964, a regression in preprocessor diagnostics
| > for C++ caused by lexing up front but deferring string interpretation
| > (which depends on the parse context) until later, so the
| > preprocessor's notion of source location does not relate to the actual
| > source location at that point.
| 
| I think this is fine.
| 
| However, I want to give the cpplib and diagnostic maintainers a chance
| to object.  If there are no objections within 24 hours, please go ahead
| and commit.

The diagnostic part of the patch is OK -- we can improve over the fine
details later.

-- Gaby