[llvm][AsmPrinter] Restore source location to register clobber warning

Since 5de2d189 this particular warning
hasn't had the location of the source file containing the inline
assembly.

Fix this by reporting via LLVMContext. Which means that we no longer
have the "instantiated into assembly here" lines but they were going to
point to the start of the inline asm string anyway.

This message is already tested via IR in llvm. However we won't have
the required location info there so I've added a C file test in clang
to cover it.
(though strictly, this is testing llvm code)

Reviewed By: ychen

Differential Revision: https://reviews.llvm.org/D102244