[Sanitizer] Fix demangling for Swift symbol names

The Swift symbol name prefix has changed from `_T0` to `_$s` as
documented here [1].  This prevents Swift names from properly being
symbolicated when using the in-process LLVM symbolizer.  The best way to
fix this seems to be to avoid the duplication of "Is this a Swift symbol
name?" here.  We can simply remove this check as `swift_demangle`
already returns null for non-Swift names [2,3].

The check was included in the initial support for Swift name demangling
to avoid superfluous calls to `dlsym()` [4].  A subsequent commit
changed this logic to retrieve the `swift_demangle` function pointer
eagerly during sanitizer initialization, but did not remove the check
[5].

[1] https://github.com/apple/swift/blob/master/docs/ABI/Mangling.rst
[2] https://github.com/apple/swift/blob/b5a8b518eae54cea997f3b0954760fc7858829f6/include/swift/Demangling/Demangle.h#L643
[3] https://github.com/apple/swift/blob/b5a8b518eae54cea997f3b0954760fc7858829f6/stdlib/public/runtime/Demangle.cpp#L656
[4] https://reviews.llvm.org/D19135
[5] https://reviews.llvm.org/D20015

rdar://62753845

Reviewers: kubamracek, delcypher, dcoughlin, samsonov, thakis

Reviewed By: kubamracek

Differential Revision: https://reviews.llvm.org/D81705