Revert "Respect Clang version override env vars."

Revert submission 2478283-master-I3cf18e7814ff52d526017e4fe34b17ab3d8f1080

Reason for revert: implementing a different way

Reverted changes: /q/submissionid:2478283-master-I3cf18e7814ff52d526017e4fe34b17ab3d8f1080

Change-Id: I1d690b6d322adc664e4c02d958f0183bbb78e669
diff --git a/cc/config/global.go b/cc/config/global.go
index 0257fa3..05dc773 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -417,12 +417,15 @@
 	exportedVars.ExportStringList("CommonGlobalIncludes", commonGlobalIncludes)
 	pctx.PrefixedExistentPathsForSourcesVariable("CommonGlobalIncludes", "-I", commonGlobalIncludes)
 
+	exportedVars.ExportStringStaticVariable("CLANG_DEFAULT_VERSION", ClangDefaultVersion)
+	exportedVars.ExportStringStaticVariable("CLANG_DEFAULT_SHORT_VERSION", ClangDefaultShortVersion)
+
 	pctx.StaticVariableWithEnvOverride("ClangBase", "LLVM_PREBUILTS_BASE", ClangDefaultBase)
-	exportedVars.ExportStringStaticVariableWithEnvOverride("ClangVersion", "LLVM_PREBUILTS_VERSION", ClangDefaultVersion)
+	pctx.StaticVariableWithEnvOverride("ClangVersion", "LLVM_PREBUILTS_VERSION", ClangDefaultVersion)
 	pctx.StaticVariable("ClangPath", "${ClangBase}/${HostPrebuiltTag}/${ClangVersion}")
 	pctx.StaticVariable("ClangBin", "${ClangPath}/bin")
 
-	exportedVars.ExportStringStaticVariableWithEnvOverride("ClangShortVersion", "LLVM_RELEASE_VERSION", ClangDefaultShortVersion)
+	pctx.StaticVariableWithEnvOverride("ClangShortVersion", "LLVM_RELEASE_VERSION", ClangDefaultShortVersion)
 	pctx.StaticVariable("ClangAsanLibDir", "${ClangBase}/linux-x86/${ClangVersion}/lib/clang/${ClangShortVersion}/lib/linux")
 
 	// These are tied to the version of LLVM directly in external/llvm, so they might trail the host prebuilts