run-clang-tidy: Use check_call instead of check_output

Summary:
Streamlines the output under Python 3.x.

Before:
```
b'Enabled checks:\n    clang-analyzer-apiModeling.google.GTest\n ...
```

After:
```
Enabled checks:
    clang-analyzer-apiModeling.google.GTest
...
```

Reviewers: cfe-commits, alexfh

Reviewed By: alexfh

Subscribers: JDevlieghere

Differential Revision: https://reviews.llvm.org/D37482

Change-Id: I6287104bc73926ae6d0f66c15c250c3cb44bee33
llvm-svn: 319148