[Builtins] Provide a mechanism to selectively disable tests based on whether an implementation is provided by a builtin library.

Summary:
If a platform removes some builtin implementations (e.g. via the
Darwin-excludes mechanism) then this can lead to test failures because
the test expects an implementation to be available.

To solve this lit features are added for each configuration based
on which sources are included in the builtin library. The features
are of the form `librt_has_<name>` where `<name>` is the name of the
source file with the file extension removed. This handles C and
assembly sources.

With the lit features in place it is possible to make certain tests
require them.

Example:

```
REQUIRES: librt_has_comparedf2
```

All top-level tests in `test/builtins/Unit` (i.e. not under
`arm`, `ppc`, and `riscv`) have been annotated with the appropriate
`REQUIRES: librt_has_*` statement.

rdar://problem/55520987

Reviewers: beanz, steven_wu, arphaman, dexonsmith, phosek, thakis

Subscribers: mgorny, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D68064

llvm-svn: 375150

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