[clang] [clang][NFC] Use BuiltinType::isFloatingPoint() in Type::isFloatingType() (PR #139035)

[Timm Baeder via cfe-commits]

tbaederr wrote:

I considered that the comparison is inlined in `isFloatingType` for performance reasons, but the git log doesn't suggest anything like that.

https://github.com/llvm/llvm-project/pull/139035