[ASan] Improve blacklisting of global variables.

This commit changes the way we blacklist global variables in ASan.
Now the global is excluded from instrumentation (either regular
bounds checking, or initialization-order checking) if:

1) Global is explicitly blacklisted by its mangled name.
This part is left unchanged.

2) SourceLocation of a global is in blacklisted source file.
This changes the old behavior, where instead of looking at the
SourceLocation of a variable we simply considered llvm::Module
identifier. This was wrong, as identifier may not correspond to
the file name, and we incorrectly disabled instrumentation
for globals coming from #include'd files.

3) Global is blacklisted by type.
Now we build the type of a global variable using Clang machinery
(QualType::getAsString()), instead of llvm::StructType::getName().

After this commit, the active users of ASan blacklist files
may have to revisit them (this is a backwards-incompatible change).