Installapi has important distinctions when compared to the clang driver,
so much that, it doesn't make much sense to try to integrate into it.
This patch partially reverts the CC1 action & driver support to replace
with its own driver as a clang tool.
For distribution, we could use `LLVM_TOOL_LLVM_DRIVER_BUILD` mechanism
for integrating the functionality into clang such that the toolchain
size is less impacted.
This introduces a basic outline of installapi as a clang driver option.
It captures relevant information as cc1 args, which are common arguments
already passed to the linker to encode into TBD file outputs. This is
effectively an upstream for what already exists as `tapi installapi` in
Xcode toolchains, but directly in Clang. This patch does not handle any
AST traversing on input yet.
InstallAPI is broadly an operation that takes a series of header files
that represent a single dynamic library and generates a TBD file out of
it which represents all the linkable symbols and necessary attributes
for statically linking in clients. It is the linkable object in all
Apple SDKs and when building dylibs in Xcode. `clang -installapi` also
will support verification where it compares all the information recorded
for the TBD files against the already built binary, to catch possible
mismatches like when a declaration is missing a definition for an
exported symbol.
