Optimize the loading of an identifier from a PCH file when given the

Optimize the loading of an identifier from a PCH file when given the
identifier's ID. In this case, we know where the identifier's entry is
located in the hash table (it starts right before the identifier
string itself), so skip the hash table lookup and read the entry
directly. The performance improvement here is, gain, hard to quantify,
but it's the right thing to do.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70078 91177308-0d34-0410-b5e6-96231b3b80d8