[WebAssembly] Support `annotate` clang attributes for marking functions.

Annotation attributes may be attached to a function to mark it with
custom data that will be contained in the final Wasm file. The
annotation causes a custom section named
"func_attr.annotate.<name>.<arg0>.<arg1>..." to be created that will
contain each function's index value that was marked with the annotation.

A new patchable relocation type for function indexes had to be created so
the custom section could be updated during linking.

Reviewed By: sbc100

Differential Revision: https://reviews.llvm.org/D150803