[clang] Return `std::string_view` from `TargetInfo::getClobbers()`

Change the return type of `getClobbers` function from `const char*`
to `std::string_view`. Update the function usages in CodeGen module.

The reasoning of these changes is to remove unsafe `const char*`
strings and prevent unnecessary allocations for constructing the
`std::string` in usages of `getClobbers()` function.

Differential Revision: https://reviews.llvm.org/D148799