Allow SVM assertions to be ignored

[cjjdespres]

If the option TR_IgnoreAssert is enabled for a particular compilation and the environment variable TR_svmAssertionsAreFatal is not set, SVM assertions will now be non-fatal even in DEBUG or PROD_WITH_ASSUMES builds. This matches the behaviour of normal TR_ASSERT assertions, aside from the possibility of overriding TR_IgnoreAssert with that environment variable.