[ThreadSanitizer] Add fallback DebugLocation for memintrinsic calls

When building with debug info enabled, some load/store instructions do
not have a DebugLocation attached. When using the default IRBuilder, it
attempts to copy the DebugLocation from the insertion-point instruction.
When there's no DebugLocation, no attempt is made to add one.

Add a fallback DebugLocation with the help of InstrumentationIRBuilder for
memintrinsics. In particular, the compiler may optimize load/store without
debug info into memintrinsics, which then are missing debug info as well.