[ADT] Add StringRef::{l,r}trim(char) overloads (NFC)

Add support for trimming a single kind of character from a StringRef.
This makes the common case of trimming null bytes much neater. It's also
probably a bit speedier too, since it avoids creating a std::bitset in
find_{first,last}_not_of.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@260925 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed