[clang-tidy] Speedup misc-static-assert.

Summary:
Speedup the misc-static-assert check by not use `stmt()` as the toplevel matcher.
The framework runs a filter on the matchers before trying them on each node and
uses the toplevel type for this.
Using `stmt()` as the toplevel causes the matcher to be run on every `Stmt` node,
even if the node doesn't match the desired types.

This change speeds up clang-tidy by ~5% in a benchmark.

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19877

llvm-svn: 268430