Reduce the number of allocations required for AST attributes. In test cases,...

Reduce the number of allocations required for AST attributes. In test cases, the max resident memory changed from 65760k to 64476k which is 1.9% improvement. Allocations in grow_pod changed from 8847 to 4872 according to tcmalloc heap profiler. Overall running time remained the same.

Patch by Eugene Kosov

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@278812 91177308-0d34-0410-b5e6-96231b3b80d8