[JITLink][NFC] Store external symbols in a StringMap

External symbols used to be stored in a `DenseSet`. An `assert` in
`addExternalSymbol` ensures that names of external symbols are unique.
However, for objects containing a huge number of external symbols, this
`assert` can be a performance bottleneck.

This patch proposes to store external symbols in a `StringMap` instead
making it significantly cheaper to check if a certain symbol name
already exists.

This issue came up while porting BOLT to JITLink (D147544): linking a
large binary using the JITLink port turned out to be about 4x slower
than the current version of BOLT that uses RuntimeDyld. This slowdown
was caused entirely by the `assert` in `addExternalSymbol`. Using this
patch, the JITLink port is slightly faster than RuntimeDyld.

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D150874