Ensure sanitizer check function calls have a !dbg location

Function calls without a !dbg location inside a function that has a
DISubprogram make it impossible to construct inline information and
are rejected by the verifier. This patch ensures that sanitizer check
function calls have a !dbg location, by carrying forward the location
of the preceding instruction or by inserting an artificial location if
necessary.

This fixes a crash when compiling the attached testcase with -Os.

rdar://problem/45311226

Differential Revision: https://reviews.llvm.org/D53459

llvm-svn: 344915