[ASTMatchers] Sprinkle some constexpr on the global matcher constructors.

This dramatically reduces the size of the global constructors we emit
for those variables in debug mode.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293518 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed