Use -fmacro-prefix-map for annot unit path in bc

[haitaka]

Clang emits source unit path with every annotation in llvm.metadata
section in bitcode.

For example, for every stdlib function annotated with
`__attribute__((annotate("introduced_in=XXX")))` bitcode will
contain a path to the unit the function was declared in.

In order to make bitcode builds reproducible, these paths should be
normalized just like `__FILE__` macro paths are.

This commit reuses `-fmacro-prefix-map` machinery for annotation
paths in bitcode.