[IR] Explicitly specify target feature for module asm (#204548)
Support specifying additional properties on module-level inline
assembly. In particular, the target features and target CPU can now be
specified as follows:
module asm(target_features: "+foo", target_cpu: "bar")
"asm line 1"
"asm line 2"
There may be multiple module inline assembly blocks with different
properties.
This is intended to fix the long standing issue where in LTO scenarios
we don't know what target features to use for parsing the module-level
inline assembly. Now they can be faithfully preserved, even when merging
inline assembly from different modules with different features.
If target_features and target_cpu are empty, we fall back to the old
behavior (which, in the backend, would be to use the default subtarget
of the TargetMachine).
Fixes https://github.com/llvm/llvm-project/issues/61991.
Fixes https://github.com/llvm/llvm-project/issues/67698.
