sys-devel/llvm: Stop enforcing external lit for tests

sys-devel/llvm: Stop enforcing external lit for tests

Modify the ebuild to no longer enforce externally installed
dev-python/lit for tests, effectively causing the build system to use
the bundled copy of lit (along with llvm-lit). While using the external
package is more canonical, its disadvantages outweight the gain.

Most notably, lit is not slotted in Gentoo and the behavior of lit
frequently changes subtly which caused us to require a major version
match between LLVM and lit. As a result, attempting to build multiple
slots of LLVM with tests enabled cause package collisions. Building both
LLVM and lit simultaneously causes a circular dependency.

Since lit comes from the same package as LLVM itself and LLVM requires
explicit python-any-r1 use anyway, using the bundled copy has no added
cost. It reduces the risk of breakage resulting from skipping llvm-lit
wrapper and solves the forementioned problems caused by depending
on dev-python/lit.

The external package is still useful for projects that do not require
LLVM sources.