[PragmaHandler] Expose `#pragma` location

Currently, a pragma AST node's recorded location starts at the
namespace token (such as `omp` in the case of OpenMP) after the
`#pragma` token, and the `#pragma` location isn't available.  However,
the `#pragma` location can be useful when, for example, rewriting a
directive using Clang's Rewrite facility.

This patch makes `#pragma` locations available in any `PragmaHandler`
but it doesn't yet make use of them.

This patch also uses the new `struct PragmaIntroducer` to simplify
`Preprocessor::HandlePragmaDirective`.  It doesn't do the same for
`PPCallbacks::PragmaDirective` because that changes the API documented
in `clang-tools-extra/docs/pp-trace.rst`, and I'm not sure about
backward compatibility guarantees there.

Reviewed By: ABataev, lebedev.ri, aaron.ballman

Differential Revision: https://reviews.llvm.org/D61643

llvm-svn: 361335

0 files changed, 0 insertions, 0 deletions