[NFC][llvm][StringMap]Extract createTable and getHashTable functions and add...

[NFC][llvm][StringMap]Extract createTable and getHashTable functions and add the inline attribute to the getMinBucketToReserveForEntries function.

1. Extract createTable and getHashTable functions.
2. Add the inline attribute to the getMinBucketToReserveForEntries function.
3. Remove unnecessary local variable HTSize.

Statements in the following order appear in  llvm::StringMapImpl::init and llvm::StringMapImpl::RehashTable, so I extracted this code into a function. getHashTable is for the same reason, it appears in  llvm::StringMapImpl::FindKey,  llvm::StringMapImpl::LookupBucketFor and llvm::StringMapImpl::RehashTable.

```
auto **Table = static_cast<StringMapEntryBase **>(safe_calloc(
      NewNumBuckets + 1, sizeof(StringMapEntryBase **) + sizeof(unsigned)));

  // Allocate one extra bucket, set it to look filled so the iterators stop at
  // end.
  Table[NewNumBuckets] = (StringMapEntryBase *)2;
```

```
unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
```

Reviewed By: skan, sepavloff

Differential Revision: https://reviews.llvm.org/D121934