Close FileEntries of cached files in ModuleManager::addModule().

While investigating why LLDB (which can build hundreds of clang
modules during one debug session) was getting "too many open files"
errors, I found that most of them are .pcm files that are kept open by
ModuleManager. Pretty much all of the open file dscriptors are
FileEntries that are refering to `.pcm` files for which a buffer
already exists in a CompilerInstance's PCMCache.

Before PCMCache was added it was necessary to hold on to open file
descriptors to ensure that all ModuleManagers using the same
FileManager read the a consistent version of a given `.pcm` file on
disk, even when a concurrent clang process overwrites the file halfway
through. The PCMCache makes this practice unnecessary, since it caches
the entire contents of a `.pcm` file, while the FileManager caches all
the stat() information.

This patch adds a call to FileEntry::closeFile() to the path where a
Buffer has already been created. This is necessary because even for a
freshly written `.pcm` file the file is stat()ed once immediately
after writing to generate a FileEntry in the FileManager. Because a
freshly-generated file's contents is stored in the PCMCache, it is
fine to close the file immediately thereafter.  The second change this
patch makes is to set the `ShouldClose` flag to true when reading a
`.pcm` file into the PCMCache for the first time.

[For reference, in 1 Clang instance there is
     - 1 FileManager and
     - n ModuleManagers with
     - n PCMCaches.]

rdar://problem/40906753

Differential Revision: https://reviews.llvm.org/D50870

llvm-svn: 340188