[llvm-ml] Implement support for MASM's extern directive

The EXTERN keyword defines external symbols in MASM.

Credit goes to epastor@ for implementing most of the logic; I (ayzhao@)
added some bugfixes and tests.

[0]: https://docs.microsoft.com/en-us/cpp/assembler/masm/extern-masm?view=msvc-170

Reviewed By: epastor

Submitted By: epastor

Differential Revision: https://reviews.llvm.org/D125273