Add clang-tidy file

This adds a .clang-tidy file to the tree, which enables a small number
of checks. The only checks that are applied are:

- readability-container-size-empty: Mostly, this makes empty string
  checks use .empty() instead of .size().
- modernize-use-override: This adds override where applicable.

This causes clang to build UHD with fewer warnings.

.clang-tidy

@@ -0,0 +1,7 @@
+---
+Checks:          '-*,misc-throw-by-value-catch-by-reference,misc-static-assert,readability-container-size-empty,modernize-use-override'
+WarningsAsErrors: ''
+AnalyzeTemporaryDtors: false
+FormatStyle:     file
+CheckOptions:
+...