[Assembler] Add test for !srcloc references in assembler diags

Summary:
clang adds !srcloc metadata to inline assembly in LLVM bitcode generated
for inline assembly in C.  The value of this !srcloc is passed to the
diagnostics handler if the inline assembly generates a diagnostic.
clang is able to turn this cookie back to a location in the C source
file.

To test this functionality without a dependency, make llc print the
!srcloc metadata if it is present.  The added test uses this mechanism
to test that the correct !srclocs are passed to the diag handler.

Reviewers: rengolin, rnk, echristo, grosbach, mehdi_amini

Reviewed By: mehdi_amini

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D30167

llvm-svn: 296465