[TypeSystem] Guard the global `ASTSourceMap` with a mutex

s_source_map in ClangExternalASTSourceCommon.cpp is unguarded 
and therefore can break in multithreaded conditions. This can 
cause crashes in particular if multiple targets are being set
up at once.

This patch wraps s_source_map in a function that ensures 
exclusivity, and makes every user of it use that function
instead.

<rdar://problem/33429774> lldb crashes after "resume_off"

Differential Revision: https://reviews.llvm.org/D35083

llvm-svn: 308993