RuntimeLibcalls: Add helper methods to insert declarations (#240444)
Add methods to get or insert the declaration of a libcall
implementation in a module with the appropriate type and
annotations. A (potentially incorrectly declared) global that already
has the name is returned unmodified
Use them in DeclareRuntimeLibcalls, which no longer merges attributes
into or replaces the calling convention of existing functions, and no
longer crashes on a global variable with the name of a libcall
function.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>