Fix handling of dontcall attributes for arches that lower calls via fastSelectInstruction (#153302)

Recently my change to avoid duplicate `dontcall` attribute errors
(#152810) caused the Clang `Frontend/backend-attribute-error-warning.c`
test to fail on Arm32:
<https://lab.llvm.org/buildbot/#/builders/154/builds/20134>

The root cause is that, if the default `IFastSel` path bails, then
targets are given the opportunity to lower instructions via
`fastSelectInstruction`. That's the path taken by Arm32 and since its
implementation of `selectCall` didn't call `diagnoseDontCall` no error
was emitted.

I've checked the other implementations of `fastSelectInstruction` and
the only other one that lowers call instructions in WebAssembly, so I've
fixed that too.

Author: dpaoliello

llvm/lib/Target/ARM/ARMFastISel.cpp

@@ -2504,6 +2504,7 @@ bool ARMFastISel::SelectCall(const Instruction *I,
   // Set all unused physreg defs as dead.
   static_cast<MachineInstr *>(MIB)->setPhysRegsDeadExcept(UsedRegs, TRI);
 
+  diagnoseDontCall(*CI);
   return true;
 }
 

llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp

@@ -912,6 +912,8 @@ bool WebAssemblyFastISel::selectCall(const Instruction *I) {
 
   if (!IsVoid)
     updateValueMap(Call, ResultReg);
+
+  diagnoseDontCall(*Call);
   return true;
 }