[lldb][test] Don't treat 'xcrun clang' as a path for finding clang++
TestPrintObjectArray.py uses `xcrun clang` as the test compiler.
This is because the test source requires some Objective-C features
that are only available in downstream clang:
```
self.build(dictionary=[...], compiler="xcrun clang")
```
However, this currently just results in us running `clang++`
instead of `xcrun clang++` to compile test sources. If the downstream
`clang++` in PATH is not Apple's downstream version, this then causes
the tests to fail with compilation errors:
```
clang++: error: unknown argument: '-fno-constant-nsnumber-literals'
clang++: error: unknown argument: '-fno-constant-nsarray-literals'
clang++: error: unknown argument: '-fno-constant-nsdictionary-literals'
```
We end up in this situation as we treat this command as a path when
trying to assemble the `clang++` command. This patch just adds a
special case for `xcrun` and we now first try the compiler (verbatim)
before falling back to the inferred compiler type.
552e68d6ac