[clang-tidy] Handle missing yaml module in run-clang-tidy.py

The Yaml module is missing on some systems and on many of clang buildbots. 
But the test for run-clang-tidy.py doesn't fail due to 'NOT' statement masking a python runtime error.

This patch conditionally imports and enables the yaml module only if it's present in the system. 
If not, then '-export-fixes' is disabled.

Differential Revision: https://reviews.llvm.org/D59734

llvm-svn: 357114