Do not use StringTableBuilder to build symbol table for .gdb_index.

Previously, we had two levels of hash table lookup. The first hash
lookup uses CachedHashStringRefs as keys and returns offsets in string
table. Then, we did the second hash table lookup to obtain GdbSymbol
pointers. But we can directly map strings to GDbSymbols.

One test file is updated in this patch because we no longer have a '\0'
byte at the start of the string pool, which was automatically inserted
by StringTableBuilder.

This patch speeds up Clang debug build (with -gdb-index) link time by
0.3 seconds.

llvm-svn: 314092