[xray] Apply default visibility to __xray_CustomEvent/__xray_TypedEvent

`__xray_customevent` and `__xray_typedevent` are built-in functions in
Clang. With -fxray-instrument, they are lowered to `__xray_CustomEvent`
(with 2 arguments) or `__xray_TypedEvent` (with 3 arguments).

xray patching is supported for shared objects, but they may contain
`__xray_customevent` and `__xray_typedevent` references that need to be
satisfied by default visibility definitions exported by the executable.

lld since df54f62, like GNU ld, catches
the scenario at link time.

Author: MaskRay

compiler-rt/lib/xray/xray_trampoline_AArch64.S

@@ -138,8 +138,9 @@ ASM_SYMBOL(__xray_ArgLoggerEntry):
   ret
 ASM_SIZE(__xray_ArgLoggerEntry)
 
+// __xray_*Event have default visibility so that they can be referenced by user
+// DSOs that do not link against the runtime.
 .global ASM_SYMBOL(__xray_CustomEvent)
-ASM_HIDDEN(__xray_CustomEvent)
 ASM_TYPE_FUNCTION(__xray_CustomEvent)
 ASM_SYMBOL(__xray_CustomEvent):
   SAVE_REGISTERS
@@ -153,7 +154,6 @@ ASM_SYMBOL(__xray_CustomEvent):
 ASM_SIZE(__xray_CustomEvent)
 
 .global ASM_SYMBOL(__xray_TypedEvent)
-ASM_HIDDEN(__xray_TypedEvent)
 ASM_TYPE_FUNCTION(__xray_TypedEvent)
 ASM_SYMBOL(__xray_TypedEvent):
   SAVE_REGISTERS

compiler-rt/lib/xray/xray_trampoline_x86_64.S

@@ -257,8 +257,9 @@ LOCAL_LABEL(arg1entryFail):
 
 //===----------------------------------------------------------------------===//
 
+// __xray_*Event have default visibility so that they can be referenced by user
+// DSOs that do not link against the runtime.
 	.global ASM_SYMBOL(__xray_CustomEvent)
-	ASM_HIDDEN(__xray_CustomEvent)
 	.align 16, 0x90
 	ASM_TYPE_FUNCTION(__xray_CustomEvent)
 # LLVM-MCA-BEGIN __xray_CustomEvent
@@ -284,7 +285,6 @@ LOCAL_LABEL(customEventCleanup):
 //===----------------------------------------------------------------------===//
 
 	.global ASM_SYMBOL(__xray_TypedEvent)
-	ASM_HIDDEN(__xray_TypedEvent)
 	.align 16, 0x90
 	ASM_TYPE_FUNCTION(__xray_TypedEvent)
 # LLVM-MCA-BEGIN __xray_TypedEvent