the `ptx_kernel` calling convention is a more idiomatic and standard way
of specifying a NVPTX kernel than using the metadata which is not
supposed to change the meaning of the program. Further, checking the
calling convention is significantly faster than traversing the metadata,
improving compile time.
This change updates the clang and mlir frontends as well as the
NVPTXCtorDtorLowering pass to emit kernels using the calling convention.
In addition, this updates all NVPTX unit tests to use the calling
convention as well.
Similar to 806761a7629df268c8aed49657aeccffa6bca449
-mtriple= specifies the full target triple while -march= merely sets the
architecture part of the default target triple (e.g. Windows, macOS),
leaving a target triple which may not make sense.
Therefore, -march= is error-prone and not recommended for tests without
a target triple. The issue has been benign as we recognize
nvptx{,64}-apple-darwin as ELF instead of rejecting it outrightly.
See the test file. At head, this crashes with
```
assertion failed at llvm/lib/Support/APInt.cpp:492 in
uint64_t llvm::APInt::extractBitsAsZExtValue(unsigned int, unsigned int) const:
bitPosition < BitWidth && (numBits + bitPosition) <= BitWidth &&
"Illegal bit extraction"
```
