Trying to fix Mangler memory leak in TargetLoweringObjectFile.

Summary:
`TargetLoweringObjectFile` can be re-used and thus `TargetLoweringObjectFile::Initialize()`
can be called multiple times causing `Mang` pointer memory leak.

Reviewers: echristo

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D24659

llvm-svn: 281718