[debuginfo-tests] Don't look for Python 3 if we already have it

LLDB already requires Python 3 on Windows, so I already configure it
that way. For some reason CMake fails to find the one that Visual Studio
automatically installs at this standard location:
  C:/Program Files (x86)/Microsoft Visual Studio/Shared/Python37_64/python.exe

CMake prefers the python on path, which happens to be python 2.7.

Reviewers: aprantl, jmorse

Differential Revision: https://reviews.llvm.org/D69684

debuginfo-tests/CMakeLists.txt

@@ -13,7 +13,10 @@ set(DEBUGINFO_TEST_DEPS
   not
   )
 
-# Wipe, uh, previous results
+# If we don't already have Python 3, throw away any previous results and try to
+# find it again.
+set(DEBUGINFO_UNSET_PYTHON3 OFF)
+if (PYTHON_VERSION_MAJOR LESS 3)
   unset(PYTHONINTERP_FOUND CACHE)
   unset(PYTHON_EXECUTABLE CACHE)
   unset(PYTHON_LIBRARY CACHE)
@@ -27,6 +30,9 @@ unset(PYTHONLIBS_VERSION_STRING CACHE)
 
   # Try to find python3. If it doesn't exist, dexter tests can't run.
   find_package(PythonInterp "3")
+  set(DEBUGINFO_UNSET_PYTHON3 ON)
+endif()
+
 if (NOT DEFINED PYTHON_EXECUTABLE)
   message(FATAL_ERROR "Cannot run debuginfo-tests without python")
 elseif(PYTHON_VERSION_MAJOR LESS 3)
@@ -46,7 +52,9 @@ else()
   set_target_properties(check-debuginfo PROPERTIES FOLDER "Debug info tests")
 endif()
 
-# Prevent the rest of llvm observing our secret python3-ness
+# Prevent the rest of llvm observing our secret python3-ness, if that wasn't
+# what was originally found.
+if (DEBUGINFO_UNSET_PYTHON3)
   unset(PYTHONINTERP_FOUND CACHE)
   unset(PYTHON_EXECUTABLE CACHE)
   unset(PYTHON_LIBRARY CACHE)
@@ -57,3 +65,4 @@ unset(PYTHON_VERSION_MAJOR CACHE)
   unset(PYTHON_VERSION_MINOR CACHE)
   unset(PYTHON_VERSION_PATCH CACHE)
   unset(PYTHONLIBS_VERSION_STRING CACHE)
+endif()