Implement caching of code-completion results for macro definitions

when the CXTranslationUnit_CacheCompletionResults option is given to
clang_parseTranslationUnit(). Essentially, we compute code-completion
results for macro definitions after we have parsed the file, then
store an ASTContext-agnostic version of those results (completion
string, cursor kind, priority, and active contexts) in the
ASTUnit. When performing code completion in that ASTUnit, we splice 
the macro definition results into the results provided by the actual
code-completion (which has had macros turned off) before libclang gets
those results. We use completion context information to only splice in
those results that make sense for that context.

With a completion involving all of the macros from Cocoa.h and a few other
system libraries (totally ~8500 macro definitions) living in a
precompiled header, we get about a 9% performance improvement from
code completion, since we no longer have to deserialize all of the
macro definitions from the precompiled header. 

Note that macro definitions are merely the canary; the cache is
designed to also support other top-level declarations, which should be
a bigger performance win. That optimization will be next.

Note also that there is no mechanism for determining when to throw
away the cache and recompute its contents.

llvm-svn: 111051