This PR is part of an effort to remove file system usage from the
command line parsing code. The reason for that is that it's impossible
to do file system access correctly without a configured VFS, and the VFS
can only be configured after the command line is parsed. I don't want to
intertwine command line parsing and VFS configuration, so I decided to
perform the file system access after the command line is parsed and the
VFS is configured - ideally right before the file system entity is used
for the first time.

This patch delays normalization of the module cache path until
`CompilerInstance` is asked for the cache path in the current
compilation context.

Resolves warnings triggered by `Woverloaded-virtual`

This is preparatory work for the implementation of `#pragma omp fuse` in
https://github.com/llvm/llvm-project/pull/139293

Not all OpenMP loop transformations makes sense to make them inherit
from `OMPLoopBasedDirective`, in particular in OpenMP 6.0 'fuse' (to be
implemented later) is a transformation of a canonical loop sequence.

This change renames class `OMPLoopTransformationDirective` to
`OMPCanonicalLoopNestTransformationDirective` so we can reclaim that
name in a later change.

The root cause of the issue is that we forgot to update the abbrev
number correctly.

The test would crash before.

This fixes the workaround added in 8a63989, so that when a fake
definition data is corrected, all redeclarations are also updated to
point to it.

Since this regression was never released, there are no release notes.

Fixes #154840

Expressions/references with 'bounds' are going to need to do
initialization significantly differently, so we need to have the
initializer and the declaration 'separate' in the future. This patch
splits the AST node into two, and normalizes them a bit.

Additionally, since this required significant work on the recipe
generation, this patch also does a bit of a refactor to improve
readability and future expansion, now that we have a good understanding
of how these are going to look.

This implements support for [named
loops](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3355.htm) for
C2y. 

When parsing a `LabelStmt`, we create the `LabeDecl` early before we parse 
the substatement; this label is then passed down to `ParseWhileStatement()` 
and friends, which then store it in the loop’s (or switch statement’s) `Scope`; 
when we encounter a `break/continue` statement, we perform a lookup for 
the label (and error if it doesn’t exist), and then walk the scope stack and 
check if there is a scope whose preceding label is the target label, which 
identifies the jump target.

The feature is only supported in C2y mode, though a cc1-only option
exists for testing (`-fnamed-loops`), which is mostly intended to try
and make sure that we don’t have to refactor this entire implementation
when/if we start supporting it in C++.

---------

Co-authored-by: Balazs Benics <benicsbalazs@gmail.com>

OpenMP 6.0 12.1.2 specifies the behavior of the strict modifier for the
num_threads clause on parallel directives, along with the message and
severity clauses. This commit implements necessary codegen changes.

Instead of querying the bitwidth and signeness of the integer literal
for every iteration, get the bitwidth directly from the `APIntStorage`
and assume the signeness to be `true` since we set the type of the
`EmbedExpr` to `Ctx.IntTy` and the type of the integer literal to that
of the `EmbedExpr`, so it should always be signed, as long as
`ASTContext::IntTy` is signed.

Before:
```
$ hyperfine -r 50 -w 3 'bin/clang -c ../../benchmarks/embed.cpp -std=c++20 -fconstexpr-steps=1000000000'
Benchmark 1: bin/clang -c ../../benchmarks/embed.cpp -std=c++20 -fconstexpr-steps=1000000000
  Time (mean ± σ):      1.796 s ±  0.090 s    [User: 0.961 s, System: 0.834 s]
  Range (min … max):    1.640 s …  2.150 s    50 runs
```
After:
```
$ hyperfine -r 50 -w 3 'bin/clang -c ../../benchmarks/embed.cpp -std=c++20 -fconstexpr-steps=1000000000'
Benchmark 1: bin/clang -c ../../benchmarks/embed.cpp -std=c++20 -fconstexpr-steps=1000000000
  Time (mean ± σ):      1.700 s ±  0.050 s    [User: 0.909 s, System: 0.789 s]
  Range (min … max):    1.637 s …  1.880 s    50 runs
```

That is roughly .1s less, or whatever, 5% or something.

The benchmark is simply:
```c++
constexpr char str[] = {
#embed "sqlite3.c" suffix(,0)
};

constexpr char str2[] = {
#embed "sqlite3.c" suffix(,0)
};

constexpr char str3[] = {
#embed "sqlite3.c" suffix(,0)
};
```
where `sqlite3.c` contains the sqlite3 amalgamation (roughly 9 million
characters).

Reverts llvm/llvm-project#146405