[Clang][ARM][Sema] Reject bad sizes of __builtin_arm_ldrex (#150419)
Depending on the particular version of the AArch32 architecture,
load/store exclusive operations might be available for various subset of
8, 16, 32, and 64-bit quantities. Sema knew nothing about this and was
accepting all four sizes, leading to a compiler crash at isel time if
you used a size not available on the target architecture.
Now the Sema checking stage emits a more sensible diagnostic, pointing
at the location in the code.
In order to allow Sema to query the set of supported sizes, I've moved
the enum of LDREX_x sizes out of its Arm-specific header into
`TargetInfo.h`.
Also, in order to allow the diagnostic to specify the correct list of
supported sizes, I've filled it with `%select{}`. (The alternative was
to make separate error messages for each different list of sizes.)
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