This patch implements the -mcmodel flag from clang, allowing the Code
Model to be changed for the LLVM module. The same set of mcmodel
flags are accepted as in clang and the same Code Model attributes are
added to the LLVM module for those flags.

Also add `-mlarge-data-threshold` for x86-64, which is automatically set
by the shared command-line code (see below). This is also added as an 
attribute into the LLVM module and on the target machine.

A function is created for `addMCModel` that is copied out of clang's
argument handling so that it can be shared with flang.

---------

Co-authored-by: Mats Petersson <mats.petersson@arm.com>

This patch does the following:

 - Consumes the PIC flags (fPIC/fPIE/fropi/frwpi etc) in flang-new.
   tools::ParsePICArgs() in ToolChains/CommonArgs.cpp is used for this.
 - Adds FC1Option to "-mrelocation-model", "-pic-level", and "-pic-is-pie"
   command line options.
 - Adds the above options to flang/Frontend/CodeGenOptions' data structure.
 - Sets the relocation model in the target machine, and
 - Sets module flags for the respective PIC/PIE type in LLVM IR.

I have tried my best to replicate how clang does things.

Differential Revision: https://reviews.llvm.org/D131533

Change-Id: I68fe64910be28147dc5617826641cea71b92d94d

This patch adds support for most common optimisation compiler flags:
`-O{0|1|2|3}`. This is implemented in both the compiler and frontend
drivers. At this point, these options are only used to configure the
LLVM optimisation pipelines (aka middle-end). LLVM backend or MLIR/FIR
optimisations are not supported yet.

Previously, the middle-end pass manager was only required when
generating LLVM bitcode (i.e. for `flang-new -c -emit-llvm <file>` or
`flang-new -fc1 -emit-llvm-bc <file>`). With this change, it becomes
required for all frontend actions that are represented as
`CodeGenAction` and `CodeGenAction::executeAction` is refactored
accordingly (in the spirit of better code re-use).

Additionally, the `-fdebug-pass-manager` option is enabled to facilitate
testing. This flag can be used to configure the pass manager to print
the middle-end passes that are being run. Similar option exists in Clang
and the semantics in Flang are identical. This option translates to
extra configuration when setting up the pass manager. This is
implemented in `CodeGenAction::runOptimizationPipeline`.

This patch also adds some bolier plate code to manage code-gen options
("code-gen" refers to generating machine code in LLVM in this context).
This was extracted from Clang. In Clang, it simplifies defining code-gen
options and enables option marshalling. In Flang, option marshalling is
not yet supported (we might do at some point), but being able to
auto-generate some code with macros is beneficial. This will become
particularly apparent when we start adding more options (at least in
Clang, the list of code-gen options is rather long).

Differential Revision: https://reviews.llvm.org/D128043