Remove environment variables from driver tests

[steven_wu]

Summary

Remove the environmental variables from Driver test

Remove some of dangerous environmental variables from clang/Driver tests. Driver tests should not rely on preset value of these variables and may actually fail because of them.
They cannot be removed in test/lit.cfg because we still need to support relocatable SDKs and other overwrite for other clang tests.

[steven_wu]

Ping. This is intended to avoid driver tests gets broken by the unexpected environment variable. They can be accidentally set by user or bots and report false positive in driver tests.

[rnk]

We already have this mechanism in test/lit.cfg. Feel free to add some more
variables.

[steven_wu]

• Add more environment variables to the blacklist

[steven_wu]

After some thoughts, I think the first patch works better to serve the purpose. Environmental variable SDKROOT can be important for other clang tests but dangerous to Driver test. It can pollute all the driver tests doesn't specify a default sysroot and causing target triple to mismatch.
My goal is to recommit r226005 which is reverted because Darwin bots has SDKROOT environment set.

cfe/trunk/test/Driver/lit.local.cfg

 config.suffixes = ['.c', '.cpp', '.h', '.m', '.mm', '.S', '.s', '.f90', '.f95',
                    '.cu']
 config.substitutions = list(config.substitutions)
 config.substitutions.insert(0,
     ('%clang_cc1',
      """*** Do not use 'clang -cc1' in Driver tests. ***""") )
+
+# Remove harmful environmental variables for clang Driver tests.
+# Some might be useful for other tests so they are only removed here.
+driver_overwrite_env_vars = ['MACOSX_DEPLOYMENT_TARGET',
+                             'IPHONEOS_DEPLOYMENT_TARGET',
+                             'SDKROOT', 'CCC_OVERRIDE_OPTIONS',
+                             'CC_PRINT_OPTIONS', 'CC_PRINT_HEADERS',
+                             'CC_LOG_DIAGNOSTICS']
+
+for name in driver_overwrite_env_vars:
+  if name in config.environment:
+    del config.environment[name]