IR: Add debug info flag string conversions

Add `DIDescriptor::getFlag(StringRef)` and
`DIDescriptor::getFlagString(unsigned)`.  The latter only converts exact
matches; I'll add separate API for breaking the flags bitfield up into
parts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230107 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed