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

[alvinhochun]

Summary

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.

[mstorsjo]

This looks good to me. If nobody wants to object to it, I’m ok with approving it myself after a couple days.

[jdenny]

Seems reasonable to me, but I'm not a windows person. Will the duplicate line be removed from the other test suites whose reviews are linked from the summary?

[mstorsjo]

In D149064#4293008, @jdenny wrote:

Seems reasonable to me, but I'm not a windows person. Will the duplicate line be removed from the other test suites whose reviews are linked from the summary?

Yes, we’ll take care to remove the duplicate lines from individual testsuites afterwards.

[mstorsjo]

There doesn't seem to be any objections to this, so I think we could go ahead and land it.

llvm/utils/lit/lit/TestingConfig.py

def fromdefaults(litConfig):
             pass_vars += [
                 'COMSPEC',
                 'INCLUDE',
                 'LIB',
                 'PATHEXT',
                 'USERPROFILE',
             ]
             environment['PYTHONBUFFERED'] = '1'
+            # Avoid Windows heuristics which try to detect potential installer
+            # programs (which may need to run with elevated privileges) and ask
+            # if the user wants to run them in that way. This heuristic may
+            # match for executables containing the substrings "patch" (which is
+            # a substring of "dispatch"), "update", "setup", etc. Set this
+            # environment variable indicating that we want to execute them with
+            # the current user.
+            environment['__COMPAT_LAYER'] = 'RunAsInvoker'
 
         for var in pass_vars:
             val = os.environ.get(var, '')
             # Check for empty string as some variables such as LD_PRELOAD cannot be empty
             # ('') for OS's such as OpenBSD.
             if val:
                 environment[var] = val