DI: Require subprogram definitions to be distinct

As a follow-up to r246098, require `DISubprogram` definitions
(`isDefinition: true`) to be 'distinct'.  Specifically, add an assembler
check, a verifier check, and bitcode upgrading logic to combat testcase
bitrot after the `DIBuilder` change.

While working on the testcases, I realized that
test/Linker/subprogram-linkonce-weak-odr.ll isn't relevant anymore.  Its
purpose was to check for a corner case in PR22792 where two subprogram
definitions match exactly and share the same metadata node.  The new
verifier check, requiring that subprogram definitions are 'distinct',
precludes that possibility.

I updated almost all the IR with the following script:

    git grep -l -E -e '= !DISubprogram\(.* isDefinition: true' |
    grep -v test/Bitcode |
    xargs sed -i '' -e 's/= \(!DISubprogram(.*, isDefinition: true\)/= distinct \1/'

Likely some variant of would work for out-of-tree testcases.

llvm-svn: 246327

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1950,7 +1950,8 @@ std::error_code BitcodeReader::parseMetadata() {
 
       MDValueList.assignValue(
           GET_OR_DISTINCT(
-              DISubprogram, Record[0],
+              DISubprogram,
+              Record[0] || Record[8], // All definitions should be distinct.
               (Context, getMDOrNull(Record[1]), getMDString(Record[2]),
                getMDString(Record[3]), getMDOrNull(Record[4]), Record[5],
                getMDOrNull(Record[6]), Record[7], Record[8], Record[9],