Allow demangler's node allocator to fail, and bail out of the entire

demangling process when it does.

Use this to support a "lookup" query for the mangling canonicalizer that
does not create new nodes. This could also be used to implement
demangling with a fixed-size temporary storage buffer.

Reviewers: erik.pilkington

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51003

llvm-svn: 340670