Set hidden attribute on lprofMergeValueProfData

[apazos]

Summary

The changes in https://reviews.llvm.org/D44847 cause load time failure
due to lprofMergeValueProfData in Android libs enabled with profile generation:

"dlopen failed: cannot locate symbol "lprofMergeValueProfData" referenced by..."

Marking lprofMergeValueProfData as hidden so the correct in-module definition
is picked by the linker.

[davidxl]

Test case?

[apazos]

A test would need to dump the contents of the symbol table and verify the hidden attribute for that symbol. Do you have something else in mind?

[davidxl]

Can you write a test case with a shared library that is instrumented and write a main program with dlopen to demonstrate the problem if the fix is not there?

[apazos]

I observed the issue with instrumented libs in Android. I have not been successful in reducing a test case that would fail on x86 linux.
I can write a test case that shows I can do a dlopen/dlsym of lprofMergeValueProfData from a different library, but with the hidden attribute set on the function, the dlsym call will fail.
Shouldn't we prevent lprofMergeValueProfData from being accessed outside a module?

[apazos]

I added a test case to verify the fix prevents lprofMergeValueProfData from another module from being accessed.

[davidxl]

lgtm

lib/profile/InstrProfilingMergeFile.c

 #include "InstrProfilingInternal.h"
 #include "InstrProfilingUtil.h"
 
 #define INSTR_PROF_VALUE_PROF_DATA
 #include "InstrProfData.inc"
 
 /* Merge value profile data pointed to by SrcValueProfData into
  * in-memory profile counters pointed by to DstData.  */
+COMPILER_RT_VISIBILITY
 void lprofMergeValueProfData(ValueProfData *SrcValueProfData,
                              __llvm_profile_data *DstData) {
   unsigned I, S, V, DstIndex = 0;
   InstrProfValueData *VData;
   ValueProfRecord *VR = getFirstValueProfRecord(SrcValueProfData);
   for (I = 0; I < SrcValueProfData->NumValueKinds; I++) {
     VData = getValueProfRecordValueData(VR);
     unsigned SrcIndex = 0;