[lldb][NFCI] Refactor TypeSystemClang::GetBasicTypeEnumeration

`TypeSystemClang::GetBasicTypeEnumeration(ConstString)` builds up a
giant `UniqueCStringMap<lldb::BasicType>` which is effectively a vector whose elements
are (ConstString, lldb::BasicType). Once sorted, we get to have fairly
quick lookups (O(log n) on average). This is fine, but we can do better
on average with an llvm::StringMap.

This also allows us to simplify the initialization code for the lookup,
no more `once_flag`!

Additionally, I removed unused declarations of related functions from
CompilerType and TypeSystemClang

Differential Revision: https://reviews.llvm.org/D152315