[clang] Add cc1 --output-asm-variant= to set output syntax

2fcaa549a824efeb56e807fcf750a56bf985296b (2010) added cc1as option
`-output-asm-variant` (untested) to set the output syntax.
`clang -cc1as

[clang] Add cc1 --output-asm-variant= to set output syntax

2fcaa549a824efeb56e807fcf750a56bf985296b (2010) added cc1as option
`-output-asm-variant` (untested) to set the output syntax.
`clang -cc1as -filetype asm -output-asm-variant 1` allows AT&T input and
Intel output (`AssemblerDialect` is also used by non-x86 targets).

This patch renames the cc1as option (to avoid collision with -o) and
makes it available for cc1 to set output syntax. This allows different
input & output syntax:

```
echo 'asm("mov $1, %eax");' | clang -xc - -S -o - -Xclang --output-asm-variant=1
```

Note: `AsmWriterFlavor` (with a misleading name), used to initialize
MCAsmInfo::AssemblerDialect, is primarily used for assembly input, not
for output.
Therefore,
`echo 'asm("mov $1, %eax");' | clang -x c - -mllvm --x86-asm-syntax=intel -S -o -`,
which achieves a similar goal before Clang 19, was unintended.

Close #109157

Pull Request: https://github.com/llvm/llvm-project/pull/109360

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