Tests should not run expressions that reference platform-specific symbols

[llvmbot]

Bugzilla Link	24490
Version	unspecified
OS	Windows NT
Blocks	#22140
Reporter	LLVM Bugzilla Contributor

Extended Description

Examples of tests that do this are:

TestCPPThis.py (calls getpid which is not on Windows)
TestExprs2 (uses the global variable environ, which is not on Windows)
TestConstVariables.py (also calls getpid)

The implementations of these tests should be changed so that they either do not reference platform specific functions, or the test should detect the platform and call the appropriate function.