Add an AST matcher for real floating-point types. e.g., float, double, long...

Add an AST matcher for real floating-point types. e.g., float, double, long double, but not complex.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261221 91177308-0d34-0410-b5e6-96231b3b80d8