[OpenMP] Set pragma start loc to `#pragma` loc

This patch adjusts `PragmaOpenMPHandler` to set the location of
`tok::annot_pragma_openmp` to the `#pragma` location instead of the
`omp` location so that the former becomes the start location of the
OpenMP AST node.  This can be useful when, for example, rewriting a
directive using Clang's Rewrite facility.  Most of this patch updates
tests for changes to locations in diagnostics and `-ast-dump` output.

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

Differential Revision: https://reviews.llvm.org/D61509

llvm-svn: 361867