We used to allocate thread-local memory on each compile.
If the compile did not happen on the same thread as ShInitialize,
we leaked the thread-local memory.

It turns out that there is no need to allocate any thread-local
memory. This patch cleans up all the unnecessary junk around TLS.

BUG=crbug.com/181691
R=kbr@chromium.org

Review URL: https://codereview.appspot.com/11679046

Conflicts:
	src/compiler/ConstantUnion.h
	src/compiler/ShaderLang.cpp
	src/compiler/Types.h
	src/compiler/ValidateLimitations.cpp