[PDB] Add the ability to lookup global symbols by name.

The Globals table is a hash table keyed on symbol name, so
it's possible to lookup symbols by name in O(1) time.  Add
a function to the globals stream to do this, and add an option
to llvm-pdbutil to exercise this, then use it to write some
tests to verify correctness.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343951 91177308-0d34-0410-b5e6-96231b3b80d8
12 files changed