[lit] Set __COMPAT_LAYER=RunAsInvoker when running tests on Windows

Windows heuristics try to detect old installer programs and run them as
elevated. These heuristics include checking for certain substrings in
file names, e.g. "patch", "update", "setup". When lit tries to run
affected executables, it produces "[WinError 740] The requested
operation requires elevation" instead. Several tests have been affected
by this issue (see https://reviews.llvm.org/D137772,
https://reviews.llvm.org/D148988, https://reviews.llvm.org/D149004).

We should set this environment variable to disable this heuristic once
and for all, instead of setting it in inidividual test suites. There
should be no lit tests that legitimately want to run as elevated.

Differential Revision: https://reviews.llvm.org/D149064