Add -misc-use-anonymous-namespace to .clang-tidy

This is at odds with the coding standard.
Quoting https://llvm.org/docs/CodingStandards.html#anonymous-namespaces
> Because of this, we have a simple guideline: make anonymous namespaces
> as small as possible, and only use them for class declarations.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D149664

.clang-tidy

@@ -1,4 +1,4 @@
-Checks: '-*,clang-diagnostic-*,llvm-*,misc-*,-misc-const-correctness,-misc-unused-parameters,-misc-non-private-member-variables-in-classes,-misc-no-recursion,readability-identifier-naming'
+Checks: '-*,clang-diagnostic-*,llvm-*,misc-*,-misc-const-correctness,-misc-unused-parameters,-misc-non-private-member-variables-in-classes,-misc-no-recursion,-misc-use-anonymous-namespace,readability-identifier-naming'
 CheckOptions:
   - key:             readability-identifier-naming.ClassCase
     value:           CamelCase