Inline function into only use.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215925 91177308-0d34-0410-b5e6-96231b3b80d8
Use std::unique_ptr to simplify this code a bit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215926 91177308-0d34-0410-b5e6-96231b3b80d8
return a std::unique_ptr from getMainBufferWithPrecompiledPreamble. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215927 91177308-0d34-0410-b5e6-96231b3b80d8