This is just a dotest check to see if we can compile a simple program that uses
libc++. Right now we are parsing the rather big `algorithm` header in the test
program, but the test really just checks whether we can find *any* libc++
headers and link against some libc++ SO. Using the much smaller `cassert` header
for checking whether we can find libc++ headers speeds up this check by a bit.
After some incredibly unscientific performance testing this saves a few seconds
when running the test suite on Linux (on macOS we hardcoded that libc++ is
always there, so this check won't be used there and we don't save any time).
Reviewed By: jankratochvil
Differential Revision: https://reviews.llvm.org/D101056
5b614eb4ea