IR: Rewrite uniquing and creation of MDString

Stop using `Value::getName()` to get the string behind an `MDString`.
Switch to `StringMapEntry<MDString>` so that we can find the string by
its coallocation.

This is part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221960 91177308-0d34-0410-b5e6-96231b3b80d8
X86: use getConstant rather than getTargetConstant behind BUILD_VECTOR.

getTargetConstant should only be used when you can guarantee the instruction
selected will be able to cope with the raw value. BUILD_VECTOR is rather too
generic for this so we should use getConstant instead. In that case, an
instruction can still consume the constant, but if it doesn't it'll be
materialised through its own round of ISel.

Should fix PR21352.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221961 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Sink some reporting logic into CoverageMapping

This teaches CoverageMapping::getCoveredFunctions to filter to a
particular file and uses that to replace most of the logic found in
llvm-cov report.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221962 91177308-0d34-0410-b5e6-96231b3b80d8