[TBDGen] Mangle symbols before putting them in the TBD

[harlanhaskins]

The linker expects to see mangled symbols in the TBD, otherwise it won't
be able to link anything. Use LLVM's mangler to mangle them.

Fixes rdar://54055049

[harlanhaskins]

@swift-ci please smoke test

[jrose-apple]

Nitpick: the alphabet

[cachemeifyoucan]

Does the test failure means swift TBDGen is not getting the compatibility version/current version correctly?

[harlanhaskins]

TAPI has started defaulting compatibility-version to 0 in its TBDs, which I think is wrong. But I don't want to stray from what TAPI is doing, since we're using llvm/TextAPI I think we should use whatever it thinks the default should be.

[harlanhaskins]

For a Swift-only framework, TAPI produced this for the HeadersOnly TBD:

--- !tapi-tbd-v3
archs:           [ x86_64 ]
platform:        macosx
flags:           [ installapi ]
install-name:    '<redacted>'
current-version: 202
compatibility-version: 0
...

[cyndyishida]

For a Swift-only framework, TAPI produced this for the HeadersOnly TBD:

--- !tapi-tbd-v3
archs:           [ x86_64 ]
platform:        macosx
flags:           [ installapi ]
install-name:    '<redacted>'
current-version: 202
compatibility-version: 0
...

This is the intended behavior, there's discussion/explanation in #27736

[harlanhaskins]

@swift-ci please smoke test