[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.

llvm-svn: 260925
2 files changed