[LLParser] Support symbolic address space numbers

This allows the LLParser to also accept "A", "G", and "P" in `addrspace`
usages. "A" will be replaced by the alloca address space defined in the
globals, "G" by the default globals address space and "P" by the program
address space. This makes it easier to write tests that use different
address space and only only vary the RUN: lines. Currently, the only
alternative is to pre-process the sources with a tool such as `sed`

Importantly, these new string values are only accepted in .ll files and
not stored in the bitcode format, so it does not round-trip via llvm-as
and llvm-dis (see newly added test).

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D138789