[C API]: Add getters for inline assembly template string, constraints, and flags

This change adds support for accessing information about inline
assembly calls through the C API, enough to be able to round-trip the
information. This partially addresses https://llvm.org/pr42692 which
points out gaps in the C API

Getters for each of the parameters to LLVMGetInlineAsm/InlineAsm::get
have been added, such that the C API now has enough surface to clone
inline assembly calls

This API currently only returns the raw constraint string via
LLVMGetInlineAsmConstraintString: it may be prudent to also expose the
parsed constraints via InlineAsm::ParseConstraints, but I wasn't sure
how that should look like. This at least exposes the information for
clients

Patch by Benji Smith. Thanks!

Differential Revision: https://reviews.llvm.org/D153185