[clang][Driver] Fix tool path priority test failures

Summary:
Failure type 1:
This test can fail when the path of the build includes the strings
we're checking for. E.g "/gcc" is found in ".../gcc_7.3.0/..."

To correct this look for '"' on the end of all matches. So that we
only match the end of paths printed by clang -###.
(which would be ".../gcc_7.3.0/.../gcc" for the example)

Also look for other gcc names like gcc-x.y.z in the first check.
This confirms that the copy of clang we made is isolated as expected.

Failure type 2:
If you use a triple like "powerpc64le-linux-gnu" clang actually reports
"powerpc64le-unknown-linux-gnu". Then it searches for the
former.

That combined with Mac OS adding a version number to cmake's triple
means we can't trust cmake or clang to give us the one default triple.
To fix the test, write to both names. As they don't overlap with our
fake triple, we're still showing that the lookup works.

Reviewers: MaskRay, stevewan

Reviewed By: stevewan

Subscribers: miyuki, JDevlieghere, steven.zhang, stevewan, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83055