[SamplePGO] Fix callsite sample lookup to use dwarf names when dwarf linkage name isn't available.

When linkage name isn't available in dwarf (ususally the case of C code),  looking up callee samples should be based on the dwarf name instead of using an empty string.

Also fixing a test issue where using empty string to look up callee samples accidentally returns the correct samples because it is treated as indirect call.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D112948